


The Scientist

by talkingsoup



Series: The Scientist [1]
Category: Undertale (Video Game)
Genre: Body Horror, Child Death, Emotional Manipulation, Emotional/Psychological Abuse, Gaster Headcanons, Gen, Mental Health Issues, Mental Instability, Original Character(s), Panic Attacks, Post-Traumatic Stress Disorder - PTSD, The Life and Times of W.D. Gaster, Trauma
Language: English
Status: Completed
Published: 2015-11-28
Updated: 2015-11-28
Packaged: 2018-05-03 17:57:33
Rating: Teen And Up Audiences
Warnings: Major Character Death
Chapters: 10
Words: 53,057
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5301182
Author URL: https://archiveofourown.org/users/talkingsoup/pseuds/talkingsoup
Summary: <blockquote class="userstuff">
              <p>Monsters have been sealed underground after the war with humanity. The new Royal Scientist, Dr. W.D. Gaster, thinks he can do something about it.</p><p>Follows the life and eventual downfall of the man who speaks in hands.</p>
            </blockquote>





	1. Something Needs to Be Done About That

**Author's Note:**

  * Translation into Tiếng Việt available: [Nhà Khoa Học](https://archiveofourown.org/works/8732044) by [MattockManiac](https://archiveofourown.org/users/MattockManiac/pseuds/MattockManiac)
  * Translation into Русский available: [Учёный](https://archiveofourown.org/works/13852626) by [Refi44](https://archiveofourown.org/users/Refi44/pseuds/Refi44)



> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

The barrier fizzled and sealed itself around the cave entrance with a hiss like fading lightning. The cave echoed with screams and cries. Monsters ran through the darkness searching for loved ones or sat crying as they tended to the wounded. Some of them were already beating fists or claws or tentacles against the barrier in helpless denial. They had lost… _everything._  Thousands of lives, and now their whole world. Trees, the oceans, the  _sun._ All gone. They were trapped, sealed underground forever.

W.D. Gaster stood quiet among the chaos around him, watching the barrier with measured calm. He couldn’t keep his hands from shaking, but the rest of him was still. He needed to  _think._  There wasn’t time for panic or grief. He needed to study the barrier now, before the humans on the other side buried the cave’s entrance. He might never have a chance to see it again. Still trembling, he reached into a pocket for his notebook and began to take notes. Eerie, pulsing light filtered through the barrier.

Seven humans had cast this spell. Seven souls. The power of the human soul was truly incredible.

“Gaster? Gaster! Oh, thank goodness!”

He blinked hard and looked up, but didn’t stop writing. He had always been good at moving his hands without needing to think about it.

King Asgore rushed toward him, Queen Toriel close behind. Asgore looked stricken; Toriel’s face was unreadable. The king clapped his hands on Gaster’s shoulders.

“Thank goodness you’re alright. I thought for sure—earlier, when you stayed behind in your laboratory…”

Gaster answered. There was the familiar hesitation on Asgore’s face as the king had to mentally translate what Gaster was saying. Gaster didn’t speak like any other monster alive. The king and queen were some of the few monsters who could understand his speech.

**“I made it just in time. Unfortunately, I wasn’t fast enough to gather any of my equipment. I shall have to start from scratch.”**

“I am just glad you’re safe, old friend.” The relief in Asgore’s voice was painful to hear. “What about your family? Did they ma—”

 **“That is not important right now.”**  Gaster’s hands shook, but he kept writing. There was no time. The light was already starting to fade as the humans buried the entrance. He gestured at the barrier, at the last glimpse of filtered light.

**“Something needs to be done about that.”**

 

***

 

Asgore named the blossoming city “Home.” He had never been good at naming things.

They built within the first cavern they could find, not far from where the cave’s entrance had been. The cavern was small. Eventually, the monsters would have to make their way deeper underground, find other caverns large enough to support the population.

For now, the numbers were still small. So few of them left. But it had been years now, and they were rebuilding, repopulating. Monsters could be resilient when they put their minds to it.

Resilience was a powerful thing. Gaster had been thinking about that a lot, lately.

“I think it’s high time we had an official science division,” Asgore said to him one day. They stood on a parapet overlooking the small city. “A team of brilliant minds to devote themselves to helping monsters adapt to our new lifestyles. To study the underground—and to begin to try and find a way out of it.”

Asgore’s eyes met Gaster’s. Not many people liked to look Gaster in the eye; he thought it must be because they found his stare…unsettling. Asgore was either too kind or too naïve to really notice. Probably both.

“I want you to lead it. I want you to be my Royal Scientist, in charge of the entire science division.”

**“Me, Your Majesty?”**

Asgore chuckled. “Who else? You’re the most brilliant of those brilliant minds I mentioned.”

It was true. That wasn’t ego; it was simple fact.

“You’d have all the resources you could need. A team. I’d only ask that you keep me up to speed on what you’re working on.” Asgore elbowed him lightly. “And come by for dinner once in awhile. We never see you anymore—Tori keeps asking about you.”

Gaster smiled faintly.  **“I am sorry, King Asgore. I have been so busy lately. I would be honored to lead the science division. And…of course, I would be delighted to spend more time with you and the queen.”**

A team. A true lab, not the makeshift one he ran in his basement. The resources to finally, maybe, make progress on his study of the barrier and its relation to souls. He had been at a standstill these last few years, making the same notes and taking the same measurements over and over.

He and the king shook hands.

“Excellent! I had a feeling you would agree,” Asgore said with a wink. Gaster noticed for the first time that the king was starting to grow a beard.

Asgore slung an arm around Gaster’s narrow shoulders and steered him back toward the castle.

“Why don’t we discuss the details over tea? Tori discovered that you can make a tea out of those golden flowers you find growing down here. It’s positively _delicious._  And you can tell me about what projects you look forward to working on.”

 **“I have far too many ideas. I fear most of them would bore you.”** Gaster smiled again. He hadn’t smiled this much in  _years._   **“But I do have one rather exciting project that I’ve been working with on paper. Monster society will need a source of power here in the underground. If we can’t utilize the wind or sun anymore, then…our energy will have to come from below. Geothermal power. Of course, we will need some kind of machine to extract it. Something massive…”**

 

***

 

The cavern became cramped sooner than anyone could have predicted. Within only a few decades, monsters were starting to trickle out of Home in search of a better place to settle. Questing parties had found a chilly cavern not far from Home; and beyond that, a massive network of rivers and waterfalls. The caverns only grew larger the deeper the monsters travelled.

After the first trickles came the flood. Soon there was a mass exodus out of Home, with the king and queen leading the way. Gaster and his scientists went with them. The old city was nearly empty in a matter of months, and the long journey through the dark began.

Gaster never missed Home even once.

On such a long journey, it stood to reason that a sort of religion began to spring up. Gaster began to overhear monsters speaking off a “prophecy” concerning the old Delta Rune. They said that someday, someone would descend from the world above to free the monsters from the underground—in one way or another. Hogwash, as far as Gaster was concerned. He had more important things to think about than silly prophecies. According to some of the more industrious monsters who had gone on ahead, there was a place beyond the waterfalls where the earth opened up. Where the fiery heart of the world was exposed. If the rumors were true, it was the perfect place to build his power station. His…Core.

 

***

 

“Phew, it really is  _hot_  here, isn’t it?” Asgore was saying, tugging at the collar of his robe. “And not a flower in sight. We should call this place…hot…Hot  Place. Lava…Level? Hm. Hot…lands?”

Gaster nodded vaguely, not really paying attention. He was surveying the area, taking in the vast,  _vast_ amounts of magma that coursed through the earth only a few hundred feet below them all. Everything was cast in an orange glow.

**“Yes, ah…that last one, I think, that would work…”**

Asgore fanned himself. “Some of the monsters are saying that there is a massive cavern a few miles ahead that should serve us all well. Beyond that, well, it seems they’ve found another edge of the barrier.” Asgore smiled a little sadly. “It seems our journey is at last almost over.”

Gaster turned to look at him.

**“I think I will stay here. Some monsters have already started to build a settlement. This place will be perfect for my Core plan. I think it is best if I build the science division’s laboratory nearby, so we can directly monitor the construction and activation of the Core.”**

It would take years to build the thing, and Gaster needed to be there for every single step of the process. There were certain…aspects to the Core that he particularly needed to supervise. When it was finished, it was going to be so much more than just a power station. But the king didn’t need to know that just yet. Even Gaster still wasn’t certain about how to do what he truly wanted to do.

Asgore looked rather pained at the news.

“I had a feeling this was coming,” he said. “You don’t want to at least accompany us? See the other end of the barrier?”

A large part of Gaster never wanted to see the barrier again. But the scientist in him knew that he had to. He gave the king’s shoulder a squeeze.

**“I will visit you in your new home. I promise.”**

Later, Gaster watched until the king, queen, and the thousands of monsters following them were out of sight. Then he turned to his science team.

**“Well, no sense in dawdling. We have a lot of work to do.”**


	2. New Hire

**Summary for the Chapter:**

> Gaster builds the Core. The First Fallen Human leaves almost as quickly as they arrive. Gaster takes on a budding young scientist.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

The sticking point, the thing that had never made sense to Gaster, was how unbelievably powerful humans were. To look at the two races side by side, you would think it was the opposite. Monsters had magic. They could summon fire, or move things without touching them, or turn earth into steel and bend it to their will, quite literally. Even the youngest monster could summon a unique bullet pattern. Very powerful monsters could even use their magic to affect the souls of others. And yet despite all of that, every monster in existence could never hope to match even a single human. Just as most humans couldn’t understand magic, humans had powers no monster could understand.

Determination. Save. And most interestingly, Reset.

He watched out one of his laboratory windows as monster teams shaped the skeleton of the Core. Monsters transformed rocks into steel. Other monsters welded things together with their fire magic. Gaster frowned.

All this power, he thought, and none of it mattered next to the power of Reset. If even a single monster could Save and Reset, they could go back and stop all of this from ever happening. Monsters would never have been trapped underground in the first place. Gaster thought of the plant monsters that had died out within only a few years of losing the sun. He thought about children who grew up without ever knowing what the sky looked like, who made wishes on glittering stones in the cavern roof above Waterfall. He thought of the construction worker who had fallen into the magma while working on the Core only yesterday, and of the letter that Gaster still needed to write to the monster’s family. He thought of his own family.

Gaster was very good at keeping his emotions in check, but sometimes…sometimes it felt like there was nothing but rage inside him.

He turned away from the window and leaned on his desk, not looking at the half-written letter. The rest of his desk was strewn with notes and blueprints and books. He had been losing all sense of organization of late. He took a breath and sat down, idly drawing a heart shape in the corner of one of the blueprints.

He couldn’t artificially reproduce a soul. He had tried, again and again and again over the years. If he was ever going to truly understand the powers of the human soul, he was going to need the real thing. But it could be thousands of years before a monster ever saw a human again. Or maybe never.

His gaze fell on the letter again. He let his head drop onto the desk.

He could just quit. Turn all his attention on the Core.

Gaster had never been good at quitting. 

 

***

 

One day a human fell into the Underground.

Gaster didn’t even have time to be excited before the king and queen announced that they would be adopting the child and raising them as their own. Gaster would never say as much to Asgore or Toriel, but he just couldn’t understand it. He  _couldn’t._  A human soul, right there for the taking, and yet they were just letting the little creature  _live_  with them. A human, whose ancestors were the ones who had driven the monsters underground in the first place.

Asgore in particular had always been soft, and Toriel loved children. The rest of the Underground was practically rejoicing. The war had been ages ago, the Core had long been up and running, and monsters were, quite frankly, resigning themselves to the rest of eternity underground. Those who had been alive during the war were beginning to die out. A human coming to live with the king and queen seemed like the first real sign of peace.

It was maddening, having a human soul be so close, and yet so far away. Gaster even met the child once. A charming, mischievous little thing, but when Gaster looked at them, he saw the faces of those human wizards who had created the barrier in the first place.

Still, it was nice to see other monsters with hope for once. And more important than that, it proved a theory that Gaster had been trying to prove for years—that there was an opening into the Underground. If the human child had fallen down, then it was only a matter of time before another human followed. Gaster would just have to wait. He was a patient monster, but…time, ironically enough, was starting to run out. He was getting old.

Seeing a human again after all this time gave Gaster some ideas. Humans were so good at surviving due to a substance they produced called “Determination.” Artificially creating a soul had proved impossible, but he had had some success with distilling Determination in the past. Perhaps it was time to revisit some of those experiments and theories.

He began drawing up blueprints in secret.

 

***

 

It was amazing how so much could change in a single day.

The human had died. The prince, Asriel, had absorbed their soul and crossed the barrier—only to return and die mere hours later. The king had queen had lost both their children in one stroke. Toriel had vanished. The hopes and dreams of the entire monster race had turned to dust. Gaster mourned with the rest of them.

Only one good thing came of the tragedy. Asgore had finally hardened his heart. He had all but declared war. Any human who fell into the Underground was to be captured and killed, their soul collected. They would need seven to break the barrier, and after that…

Gaster wasn’t very concerned with the “after.”

It was only a matter of time.

 

***

 

So few monsters pursued the sciences these days. Certainly, Gaster’s science division had grown from a tiny handful of scientists to a fully-staffed laboratory, as well as an entire team dedicated to maintaining the Core. But keeping the numbers up was always a challenge. Every year, the number of applicants wanting to work for the famous W.D. Gaster shrank a little. This year there were only twelve, and only five of those were even worth considering.

The ones that got this far got a direct interview with Gaster himself. This was when Gaster always let his natural intimidating qualities shine. If a potential hire couldn’t handle Gaster then they had no place on his team.

The first two were decent enough. A flustered fire monster with a chemistry background; a young and eager ape monster fresh off his biology PhD. Both held up under scrutiny. The third was a slime monster who trembled in a rather unsettling way the entire time and said maybe a total of twenty words. The fourth was just a Temmie who had gotten past security.

The fifth had Gaster groaning internally as soon as he walked in. He was a skeleton and oddly short, wearing a rather cheeky grin and a rumpled coat. Nothing irritated Gaster quite like slovenliness, and the skeleton had the air of someone who slept twenty hours a day and regularly forgot his lab notes.

Still, Gaster stood up from his desk to shake the skeleton’s hand. He glanced down at the kid’s file.

**“You must be Sans.”**

The skeleton grinned up at him.

“That’s me. It’s an honor to meet you, sir. Let me guess—I’m shorter than you thought I would be?”

Gaster raised an eyebrow. No one ever really…tried to joke with him. Most monsters thought he had no sense of humor at all. Sans’s voice was a bit odd as well, not entirely unlike Gaster’s own.

 **“…Perhaps a little,”**  Gaster said after a moment.  **“Of course I mean no offense.”**

Sans waved a hand. “Nah, don’t worry about it. I get it all the time. You should see me next to my brother. He’s so tall, people can’t even believe we’re related.”

Gaster didn’t think he had met a monster this casual since Asgore. And in a job interview of all things. It was almost like Sans didn’t care whether he got the job or not.

**“Please take a seat.”**

They sat. Sans managed not to slouch.

 **“Why don’t we start by you telling me why you want to join the science division?”**  Gaster said, flipping his notebook to a fresh page.  **“And please let me know if I speak too quickly. I am aware that my speech patterns can be difficult to follow.”**

“Yeah? I hadn’t really noticed.”

 Gaster blinked at him. Sans shrugged.

 “I’m sort of used to people who talk a little differently.”

 He was continuing before Gaster had time to be surprised.

“As for why I want to join…well, I love science. I love being able to find questions and answer them. But that’s kind of a clichéd, dishonest answer, isn’t it? If I’m being honest, there’s two big reasons. The first is that I want to be able to work with you. I’m sure you get this a lot, but I’m a huge fan of your work.”

Gaster had to physically stop himself from rubbing his forehead. This old song and dance…

  **“I do get that a lot.”**

“I wrote my thesis on the Core. And I’ve read all your books, though I—”

 **“Flattery won’t get you anywhere with me, Sans,”**  Gaster interrupted. **“Everyone who comes through my doors has read my books. What exactly sets you apart from any of them? I need dedicated scientists on my team. Not brown-nosers.”**

Sans paused and scratched the side of his skull. Finally it seemed the cocky kid had been thrown off.

“Well…I guess it’s a good thing I don’t have a nose then, huh?” Sans flinched just slightly when Gaster narrowed his eyes. “Sorry, sir. That was a bad joke. I don’t mean to sound disrespectful. I have read all your books, sir, but I’ve always been more interested in your earlier work. You wrote a very thought-provoking series of papers about human souls some time ago. Your theories on the ability of Save and Reset in particular were fascinating.”

Gaster went still, watching Sans narrowly.

**“I wasn’t aware they were still teaching those papers at the university.”**

“Oh, they’re not. They teach from your books, mostly.” Sans hesitated again, lacing his fingerbones together on his lap. For once he looked thoughtful. Almost serious. “That’s the second reason I want to work for you. Humans and their abilities have always fascinated me. Everyone in the Underground thinks that we’ll need seven human souls to break the barrier. But with a power like Reset…it’s essentially time travel. With a power like that, we could fix everything before it went wrong. We wouldn’t have to wait for seven humans to randomly fall down here. And those seven random humans wouldn’t have to die.”

Sans chuckled a little.

“Obviously, that’s a little far-fetched. But the more immediate uses for powers like Save and Reset are almost infinite. It could change everything for us. I don’t know if you are still researching things like that. I know those papers are from years ago, and you’ve likely moved on. But I’ve developed some theories…even some experiments. If I could work for you, Dr. Gaster, I’d have the opportunity to test those theories. It would be nice to be able to change things for the better down here. Or at least to try.”

Gaster folded his hands and propped his chin on them, peering at Sans with his most neutral of expressions. Searching for some indication of dishonesty. Gaster’s papers on Save and Reset were years old, and hadn’t been well-read even at the time. The subject was esoteric, to say the least. These days, people were much more interested in the Core or in Gaster’s research into methods for capturing human souls. He had almost completed a containment device for housing souls, so that human souls could be transferred without the risk of a monster absorbing them. Asgore, of course, was very interested.

Meeting someone who cared more about Gaster’s pet projects than all the rest of it seemed too good to be true.

 **“I have been trying to find time to continue my research into Save and Reset,”**  he said finally after the silence had stretched on long enough.  **“The subject is rather near and dear to me. But in the last few decades, the Core and research into human soul collection has taken priority.”**

Sans looked hopeful for a moment. Gaster leaned across his desk.

 **“But what exactly can you bring to the table, Sans? How exactly do you think you can help me with that kind of research?”**  He tapped Sans’s file with a finger, without breaking eye contact.  **“Your file says that you dropped out less than a year into your PhD.”**

“That’s true,” Sans said evenly. “I know it doesn’t look very good on a job application. But I don’t regret the decision. School…never really agreed with me.”

Gaster had heard all this a thousand times before. Some of the scientists on his team had similar views on education, or were genius types who were self-taught. He found that half the time, though, people like that weren’t nearly as smart as they thought they were.

 **“I thought you were trying to avoid clichéd answers, Sans,”**  Gaster said with a faint smile.

“Oh…it’s not like that, sir. I mean…sure, school was pretty boring. But it was more that…the amount of work I had to do, for such little payoff, it didn’t seem worth it. Staying awake for days on end working on projects I didn’t care about…not seeing my brother for months and months. Family is important to me, Dr. Gaster. It’s not that I don’t like working hard on something—though my brother would tell you I’m lazy.”

He chuckled a little and looked away, out the window that granted a view of the Core in the distance.

“It’s just…it was a bad situation, and it was changing my views on what was important. I decided it wasn’t worth it. I’d rather work with people I respect, on projects I’m passionate about, and still be able to go home at the end of the day and see him. Heh. He gets cranky without a bedtime story.”

A younger brother, then. Gaster glanced away for a moment, eyes sliding out of focus. He’d had a younger brother once. A very long time ago.

“That’s not to say that I won’t be committed to my work here if you did hire me, sir,” Sans continued. “Like I said, it’s not the workload that’s the problem. If I worked here, I’d—” The cheeky grin was back. “—really put my  _backbone_  into it.”

Gaster stifled a snort, but couldn’t quite keep a smile from his face.

**“When can you begin?”**


	3. A Bright Cyan

**Summary for the Chapter:**

> The first human soul is acquired. The team begins to research Save and Reset, and Sans comes up with an idea.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Sans fit in well. He was friendly with the science teams and even with the Core staff, who didn’t interact with the rest of the science division much at all. He was smart, easily one of the smarter members of the division, but was surprisingly modest about it. His jokes continued to be unrepentantly terrible, but they always managed to lighten the mood. Gaster couldn’t help but notice that the whole science division had gotten a bit…warmer since Sans had joined.

More importantly, and despite Gaster’s reservations, Sans was quickly proving himself vital to their work. He helped complete the prototype of the soul storage containers that the division had been working on for months now. With that major project completed, all of them could move onto other research, or return to projects that had been delayed. King Asgore was no longer breathing down their necks, which meant that Gaster could at last refocus on Save and Reset.

 **“’Save’ is the ability to create a fixed point in time and space to which one can return at will,”**  Gaster said as he unrolled a blueprint onto his desk. Sans leaned down to study it closer.

**“’Reset’ is the actual ability to return to that point, as well as the ability to essentially ‘start over’ from an even earlier point. Both of these, and in fact all of the abilities of humans, stem from their sheer force of will. Their Determination. If we were truly to unlock Save, Reset, and all the rest of it, we would need some way of instilling true Determination in a monster. It is extremely difficult to artificially produce Determination. I believe that actually extracting it from a human soul would be vastly more efficient. That is what this machine is designed to do.”**

Sans nodded, tracing some of the equations on the blueprint.

“Seems like the only piece that’s missing is a human soul. Too bad they’re not readily available, huh? Anyway, this design looks pretty feasible. Dr. Betas in engineering would go wild for this.” Sans grinned up at Gaster. “And it’s shaped kind of like a skull. I love it.”

Gaster snorted.  **“I had a feeling you would. So your vote would be for Dr. Betas to head up construction, then?”**

“Sure. He said he’s been bored and looking for a challenge lately.”

Gaster rolled up the blueprint and slipped it back into its case. He studied Sans for a moment.

**“You really do talk to everyone here, don’t you, Sans?”**

The skeleton shrugged. “Well, yeah. Don’t you?”

 **“I make an effort. But I believe that people find me too intimidating for small talk.”**  It didn’t help that most monsters found it so difficult to even understand what Gaster was saying. Meeting strangers was always a nightmare. Gaster was convinced that half of his own staff never fully knew what he was saying.

“I don’t know about ‘intimidating,’” said Sans, casual as ever. “Imposing, maybe. You are  _very_  tall. How’s the weather up there, anyway?”

Gaster made a face.  **“King Asgore is taller, and I don’t believe anyone finds him very imposing. Perhaps you are projecting.”**

Sans laughed. “Maybe. Maybe I have a  _bone_  to pick with all the tall monsters in the world. Anyway…I can pass those blueprints off to Dr. Betas if you want. The extractor should be—”

There was a hurried knock on Gaster’s office door.

**“Come in.”**

A snake monster from the physics team pushed their way in, breathing fast.

“Dr. Gaster! I’m sorry to interrupt, but it’s important. The Royal Guard, they’ve—they found a—sir, a  _human_  has appeared in the Underground.”

Gaster froze.

**“…What? Where?”**

“In the forest, not far from the Ruins. Apparently a Snowdrake found it and—”

 **“My god.”**  Gaster rounded his desk, grabbing his coat from its hook.  **“Where is the human now? Has it been captured? Is Snowdin safe?”** The idea of a human running unchecked through the Underground filled Gaster with terror. Never mind the possibility of the soul being lost, or accidentally absorbed—even a single human could cause unimaginable destruction.

“Sir, the human…it’s already dead.”

Gaster couldn’t help a surge of relief.

**“And the soul?”**

“It’s still there. The Royal Guards stationed in Snowdin haven’t gotten their supply of soul containers yet, so they’re just trying to keep people away from it. I’m afraid that’s all I know.”

Gaster picked up his bag. He needed to get there as soon as possible, before anything happened to the soul.

 **“Let them know I will be arriving shortly.”**  The snake monster nodded and dashed away.

“Should I, uh…?”

Gaster turned. He had almost forgotten that Sans was still here. He considered the skeleton for a brief moment.

 **“Come with me,”** he said finally.  **“Grab your warmest lab coat. We’ll take the ferry.”**

***

 

The Royal Guard had cordoned off the area, which was better than Gaster had expected. The guards in this region were mostly dogs. Eager, but…not very skillful. The human’s body lay at the bottom of a small embankment, the soul hovering just above it and giving the snow a bluish sheen.

“I didn’t think it would be so…small.” Sans’s voice was rather quiet. He was staring at the body with a strange expression.

 **“This one was a child.”** Gaster eased his way down the snow to the bottom, pulling a soul container out of his bag.  **“I would guess maybe four years old.”**

The child was facedown in the snow, a little girl wearing a dress that was better suited to the warmth. Perhaps it was summer up on the Surface; it wasn’t like seasons mattered down here. The girl was missing a shoe, and had one pigtail tied with a ribbon. The other ribbon must have been lost somewhere. Gaster didn’t see any injuries—the child must have frozen to death.

“Just a kid?” Sans looked away. “Jeez.”

 **“Even the children of the species can pose a threat.”** Gaster frowned as a memory rose unbidden in his mind. During the war, he had once seen a child about this size kill a monster the size of a horse with one blow.

Determination was an incredible thing. And a terrible one.

“I kind of…objectively knew that a dead human would leave behind a body instead of dust. But I guess seeing it for real…”

Gaster looked up at him. The skeleton’s hands were shoved into his coat pockets, but he was fidgeting. Gaster had never seen Sans so uncomfortable.

 **“I know it is not pleasant, Sans,”**  Gaster said as gently as he could manage. He unfastened the soul container and slowly eased the ends around either side of the glowing soul. The light was warm, a bright cyan. A small part of Gaster wanted to just absorb the soul. The  _things_  he could accomplish with a power like that…

He snapped the container closed around the soul and resealed it.

 **“In fact, it’s rather horrible to see an innocent child in this state,”**  he continued, slipping the container back into his bag.  **“But you should accustom yourself to the sight of such things. Our work with human souls may well take us down some unsavory paths. If you can’t stomach that, then—”**

“Well, I can’t stomach much of anything, can I?” Sans said, putting on a grin and giving an expansive shrug. “Don’t worry about me. I was just a bit taken aback. So…we finally have a human soul.”

 **“Finally,”**  Gaster said as he made his way back up the embankment. Finally. _Finally._

“It’s about time,” Sans said, offering Gaster a hand. Gaster smiled openly as Sans helped pull him up.

“What happens to the body, though?”

Gaster glanced back down at the child. Some of the Royal Guards were already moving in to collect her.

**“The guards will bury her somewhere. Dogs are good at burying things. Let’s get back. We shouldn’t waste a moment in getting started.”**

***

Work progressed quickly. Gaster and his team had spent so long preparing for an eventual human soul that everything was ready to go as soon as the soul made it back to the labs. The science teams were practically lining up to gain access to the soul for study and experimentation. More was accomplished in the next few months than had been done in years, maybe not even since the Core’s construction. The soul passed from department to department; from technical engineering to magical engineering, from chemistry to biology, from particle physics to the medical wing.

Their studies into the soul allowed for all manner of advancements for the Underground. Most notable was what the magic engineers had uncovered—that the human soul could actually augment a monster’s magic, even without needing to absorb it. Soon half of the Underground was interested in learning what became known as “blue attacks,” which allowed monsters to easily attack any moving object with their usual bullets. Gaster posited that other human souls would have other kinds of abilities of a similar nature, and that perhaps the soul’s color corresponded to the effect it had on magic.

Naturally, at the end of the day, the soul always made it back to Gaster. He spent late nights at the lab experimenting with the soul. At first, Gaster conducted his research alone. Before long, the lizard-like Dr. Betas had joined him. True to Sans’s recommendation, Dr. Betas was positively thrilled at the idea of building the Determination Extractor, and had a prototype completed in a matter of months. Sans also began to return late to the lab after putting his brother to bed. The three of them eventually became their own science team, their focus always on unlocking the true potential of a human soul. It had been awhile since Gaster had worked so directly with other scientists; it wasn’t a bad feeling. Almost like having…friends.

They were in Gaster’s private lab in the basement one night, going through the week’s notes beneath the shadow of the Determination Extractor. The thing looked rather menacing in the dim light. Gaster flipped through the papers on his clipboard, frowning at a series of graphs. Sans was dozing at his desk, despite being surrounded by empty coffee cups. Dr. Betas snacked on popato chisps, drumming his claws on his own desk.

The human soul glowed faintly from where its container had been fastened to the wall.

 **“These findings seem pretty conclusive,”** Gaster said, breaking a long silence and dropping the clipboard onto his desk. He rubbed his forehead.  **“This soul might have the usual wealth of Determination, but there’s no trace of any Save power. It looks like the human never ‘Saved’ while they were alive—or perhaps, it simply didn’t have the ability at all.”**

“Mmm,” Sans agreed faintly without opening his eyes. “Seems like it must vary from human to human. I guess it explains why her soul didn’t reset after dying. That’d be the first thing I’d do, if I died suddenly. Just reset.”

Dr. Betas heaved a sigh, balled up a chisps bag and tossed it at the nearest trash bin. It missed.

“At least the Determination experiments are going well, r-right?” Dr. Betas had a nervous disposition and a bit of a stutter. “The test mice have been displaying some really fascinating behaviors. We might be able to move on to monster subjects soon.”

Gaster looked down at his hands. He very much hoped it was sooner rather than later. He had managed to keep it a secret so far, but he could feel it every day. Sometimes his hands went numb for hours. Sometimes he couldn’t breathe. He wasn’t a Boss Monster, and he had lived such a very long time already.

He was dying and he just…he couldn’t  _accept_  it. He had too much still to do.

He folded his hands on the desk and looked at the other two. For a moment he thought he saw Sans watching him, but when he looked again, the skeleton still had his eyesockets closed.

 **“It seems that our true work on Save and Reset will have to wait for another human soul to come along.”**  Gaster leaned back in his chair, stretching.  **“In the meantime…perhaps it’s time we started thinking outside the box.”**

Sans shifted and yawned, then finally opened his eyes a crack.

“Actually, on that note, I have had some ideas. All this work on the human soul, creating blue attacks and such…it got me thinking. We can utilize a soul’s power without needing to actual use the soul’s power ourselves. And there’s some evidence that Determination might prove toxic to a monster.”

Dr. Betas nodded, face grim. “The tests are i-inconclusive for now, but there’s always that chance.”

“But we might be able to use Save and Reset without needing Determination,” Sans continued, rubbing his skull. He picked up a file and handed it over to Gaster. “It’s implementing the same idea as the Core folks are using to power those blue lasers over there. Why not build a  _machine_  that can channel Save and Reset, without needing the soul as an immediate power source?”

Gaster flipped through the file. Sans’s handwriting was as sloppy as ever, but the notes and equations looked promising. He had even doodled some initial designs, though he was anything but an engineer. The machine was rather pod-shaped.

 **“A time machine,”**  said Gaster.

“Essentially.” Sans grinned sleepily. “Pretty cool, right?”

**“Pretty… ‘cool.’”**


	4. Determination

**Summary for the Chapter:**

> Sans reveals his 1 HP issue. The orange soul is acquired. Gaster gets desperate.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Things had settled comfortably back into a human-free lull in the Underground. While work on Save and Reset was on hold—no surprises there—the Determination experiments and other research continued apace. Gaster had turned all of his attention to the work with Determination. Progress was slow. Too slow. Gaster could feel desperation creeping up on him.

It was late one night after the rest of the staff had gone home, and Gaster was at a computer in the basement lab, compiling data from the latest rounds of tests. The only other person there was Sans. Dr. Betas was out of town for a few days and had roped Sans into keeping an eye on the lab mice. Both scientists were quiet, Gaster watching the computer screen with tired eyes and Sans was patrolling the cages, ticking off a checklist. He occasionally muttered about it being “intern work,” but Gaster ignored him.

Something on the computer dinged. Gaster rubbed his eyes and peered at the alert that had popped up.

 **“Looks like the magic regulating the temperature for the mice is running low,”** Gaster said with a sigh.  **“One would think we never paid our magic bill around here. Sans, do you mind?”**

“Hm? What?”

Gaster rubbed his eyes again, debating whether to get a fifth cup of coffee.  **“Do you mind refilling the magic gauge? It’s there on the wall.”**

“Oh, uh…sure.” There was a pause. “How?”

Gaster waved a hand.  **“Using magic, one would imagine, yes? Shooting a few bullets into it usually works.”**

There was a longer pause this time, and no characteristic sound of monster bullets.

“I, uh. I can’t. Actually.”

Gaster finally looked up, turning in his office chair. He did not have the energy for Sans’s jokes right now.

**“What do you mean, you can’t? I’m not asking you to power the Core, Sans.”**

Sans had an odd look on his face, somewhere between amused and cornered.

“I mean that I’m…not good with magic. At all.”

Gaster got reluctantly to his feet, not sure where this joke was going. A monster not being good at magic was like a flame not being good at burning things.

**“I fail to see the humor in this joke, but if you are going to be difficult…”**

He crossed the room to where a small tube emerged from the wall near the mouse cages, an indicator light above it blinking slowly red.

“I’m actually not trying to be… _humerus_ for once. Heh…okay, that was bad.” Sans rubbed the back of his skull. “I’m weak, Gaster.  _Incredibly_  weak. Haven’t you noticed? My soul is basically held together with stubbornness and humor. And my boundless charm, of course.”

Gaster had stopped at the temperature gauge and was staring at Sans. The skeleton still looked rather…pinned down.

“Why do you think I’m so short? Why do you think I haven’t used magic even once since you hired me? I—look. This is pretty much all I can do.”

He held out his hands, palms up, and squeezed his eyesockets shut. For a moment he looked almost pained. Then two tiny, rotating bones appeared floating above his hands. Sans inhaled sharply.

“This and some minor telekinesis, but it’s really unstable. Half the time it just doesn’t even work.” The bones fizzled out of existence and Sans shoved his hands in the pockets of his lab coat. “When I was a baby bones, I was sick all the time. I’d stay in my room and read about science, history. I’m better now, but I still have a…bad time using magic. I’m pretty sure all of my magic is being spent on keeping me alive.”

Gaster stared at Sans, processing what he was hearing. Sans was looking everywhere but at him.

 **“What you are telling me,”**  Gaster said quietly,  **“is that you have absolutely no way of attacking an enemy? Or of defending yourself?”**

He spread his hands and summoned some of his own bullets, watching them spin and hover in the air. They were shaped like hand signs and other symbols.

**“So if I shot these at you, or accidentally lost control of them, you would die in a matter of seconds?”**

Sans visibly tensed, his eyelights fixed on Gaster’s bullets. Gaster felt a little sick.

“N…No,” he said after a moment. “I got good at dodging. I’m  _very_  good at dodging. And…you wouldn’t do that.”

Gaster sent the bullets into the magic gauge on the wall. The light changed to green. The relief on Sans’s face was painful to see.

 **“I’m sorry. I didn’t mean to sound so…”** Gaster shook his head and rubbed at his forehead.  **“But I really don’t appreciate secrets. My god, Sans, why didn’t you tell me? A single lab accident and—”**

“I mean, yeah, a lab accident, or a rock falling from the cavern ceiling, or drowning in one of the waterfalls, or falling into the Core…yeah, any of those could kill me, and they could also kill anyone else.” Sans shrugged, a grin easing its way back onto his face. “I’m not made of glass, Doc. I’ve lived my whole life like this. It’s not a big deal. People tend to get…funny when I tell them, but it’s not like I hide it. I just don’t make a fuss about it.”

 **“You outright lied on your job application,”** Gaster said, glaring down at him.  **“You said that you had no medical issues that could impede your work.”**

“I didn’t lie…” Sans tapped his fingers together. “I just…left some things out. It’s not a medical issue, and it doesn’t impede my work. When have you ever seen it impede my work?”

**“Your sleep patterns. I suppose this explains why you are constantly falling asleep at your desk, yes?”**

“That’s just me being a lazybones. If I thought I was gonna fall asleep doing something important, like…mixing chemicals or something, I wouldn’t have—”

**“Important? What, exactly, gave you the indication that anything we do here is not important?”**

Sans sighed. “Come on, that’s not fair. You know what I meant.”

**“You lied to me, Sans.”**

Sans looked up at him, his expression shifting for the faintest moment into something sharp.

“Doc…it’s not like you haven’t been keeping secrets, too.”

Gaster was shocked into silence. A soul held together by stubbornness, humor and  _sheer nerve_  was more like it.

**“That’s an interesting way to speak to your boss, Sans.”**

“I don’t…mean any disrespect, D…sir. But it’s the truth, isn’t it? I don’t know if anyone else has noticed, but I’ve always been observant. Your hands were shaking at the meeting yesterday. And last week, I heard you wheezing in your office. When I knocked you pretended everything was fine. I’ve been noticing it for months.”

Gaster’s hands were shaking, but not from weakness this time.

**“Sans…”**

“You’re sick, aren’t you?” Sans looked down, hands deep in his pockets, shoulders hunched. “No…more than that. You’re dying.”

Silence.

Gaster could deny it. He could yell at Sans. The skeleton had no right to know such a thing. But he said nothing. He couldn’t think of a thing to say.

Sans pressed a hand to his face briefly before shoving it back in his pocket.

“I’m sorry. Obviously you have a reason for keeping it secret. It’s private. Don’t worry—I’m not going to spread it around.”

Gaster sighed and leaned back against the tiled wall, letting himself feel just how exhausted he was for the first time that night. Not just exhausted.  _Old._  Worn out.

**“I…appreciate your discretion.”**

“Is there…anything that can be done?”

 **“I have lived a very long time, Sans.”**  Gaster looked out the doorway. Across the hallway lay the shadow of the Determination Extractor.  **“At this point there is only really one thing that can be done.”**

Sans looked at the mice in their rows of cages. Most of them were asleep. The ones who weren’t were going about their mousey business in a very calculated manner. These days, they didn’t act like animals much at all.

“The more we study Determination, the more I worry about it. Dr. Betas agrees with me. If magic is the glue that keeps a monster from falling apart, then that’s what Determination is for humans. I just don’t think mixing two types of glue is going to make anything but a mess.”

 **“Science is about taking risks, Sans. But you shouldn’t worry. I have some time left, and I am not a fool.”** Gaster closed his eyes.  **“But you must understand. I need to see this through to the end. I need to see monsters freed. I remember the war. I remember when we were sealed underground. I remember the Surface. I am one of the last monsters left alive who still remembers. You have no idea, Sans. You have absolutely no idea what it was like.”**

Sans was quiet for a moment before saying, “I guess I don’t.”

Gaster didn’t answer. He hadn’t talked about these sorts of things in ages. Perhaps with Asgore, but he couldn’t remember the last time they had even had tea together. The king had changed so much since Asriel’s death, and the castle always felt empty without Toriel’s laughter. It was strange, having someone to confide in again.

**“Again, I…appreciate your discretion.”**

He couldn’t have Sans spreading all this around. It was far too late in life for people to think of Gaster as emotional.

“Don’t worry.” Sans chuckled a little. “Though if you keep talking to me like this, you might have to stop thinking of me as an employee and start thinking of me as…a friend. Heh. But it’s really bad news to try to be friends with your boss, right?”

Gaster covered his face with a hand and laughed. He hadn’t let himself truly laugh in a long time.

**“Why not…colleagues?”**

Gaster offered a hand. Sans grinned and shook it.

“It’s pretty unprofessional. Which is good, cause that’s right up my alley.”

 **“Color me unsurprised,”**  Gaster said, standing up from the wall. He turned to face Sans fully.  **“I think I will leave the rest of the data to compile overnight. I could use a drink.”**

“Yeah? Same. Hey, I know a great place in Snowdin.”

**“A bit of a trek, but I am up for it if you are.”**

Gaster put his computer in sleep mode and headed out the door, Sans close behind.

**“You know…perhaps, once the experiments are finished and we know that it’s safe, Determination could benefit us both.”**

Sans raised a brow ridge. “Yeah? You thinking of turning me into a science project, Doc?”

**“Of course not. But I may have a few ideas.”**

***

 

Gaster couldn’t believe his ears when they told him another human had been spotted in the Underground. It had been less than a year since they had acquired the light blue human soul. Comparatively, there had been an almost decade-long gap between the first fallen human and the second. This new human had been spotted near Snowdin as well. There must have been a large hole into the Underground in that region—perhaps back in the Ruins where the original Home had once stood. It made sense. Perhaps the first cave that the humans had driven them into was starting to come unburied.

The human was currently missing, possibly hiding out in Snowdin. In all likelihood, the human was hiding in plain sight. Only a tiny handful of living monsters even knew what a human looked like. It didn’t help that a good portion of that handful were dog monsters from the Royal Guard, who had notoriously bad memories.

The stress of waiting was bad for Gaster’s already poor constitution. The human might not be a child this time. Children were dangerous enough, but an adult—and an adult who had only ever heard of monsters in stories, as humans didn’t live nearly long enough for anyone to be alive to remember—an adult would have intent behind their fear. Monsters were very vulnerable to bad intent.

But Gaster couldn’t exactly assist with the search. He spent the majority of those days pacing around his office instead, worrying. Sans occasionally pried him away at nights for a round of drinks at Grillby’s, but that was mostly an excuse to go to Snowdin and keep an eye out.

Finally, after what felt like an eternity, Gaster received word that the human had been captured. Later that day, a guard arrived with a delivery: a bright orange soul, safely housed in a container…and a large box.

“It fought back,” the guard explained, expression grim. “Killed one of the new recruits…poor kid had just joined up. She was so excited…at least I can tell her family that she died a hero.”

The human’s body was battered. It was still a child, but quite a lot older than the one before; a teenager, Gaster thought. Its face was tight with a final expression of anger and ferocity. Determination. Though it hadn’t been enough this time.

**“Why bring me the body?”**

“Sorry? Come again?”

Gaster spoke slower.  **“Why bring the body to me?”**

“Oh. King Asgore wants the bodies of any humans brought to the castle, where they’ll be interred and honored for their sacrifice. But he said that you might want to study the body first.”

Gaster nodded and replaced the cover on the box.  **“Thoughtful, but unnecessary. An empty human body has little use.”**

“Fair enough. I’ll see it safely delivered to the castle, then.”

**“Thank you. And I am sorry for the loss of your recruit. My condolences to her family. I will make sure that her sacrifice is not in vain.”**

The guard gave a solemn nod. “Thank you for that, Doctor. You know, I knew humans were vicious, but this one…I don’t know if ‘vicious’ is the word I would use. It almost seemed like a cornered animal. Desperate.”

**“’She.’ I believe this one was female.”**

“Yeah? Kind of hard to tell.” The guard frowned at the box. “She kept screaming at us about a missing sister, accusing us of eating her. Imagine that! She was crying even while she kept fighting us. It’s kind of sad. We just couldn’t reason with her. A real shame.”

Gaster held the soul container in his hands and stared into the fiery, orange light.

**“A shame.”**

***

 

One night while working late, Gaster collapsed in his office.

It was sudden. As if his body had simply stopped working. He lay on the office floor, gasping for breath, struggling to rise.

Too soon. No, this was too soon. He was  _so close._ The Determination experiments were nearing completion, the work with the orange human soul had borne all kinds of new fruit, but it had yielded no evidence of Save power. They were stuck again on Save and Reset, which meant that the Determination experiments  _had_  to come through. He was  _so close._  And yet here he was. Fallen down. He could almost feel his insides dissolving.

Not yet,  _not yet._  Gaster managed to hook his fingers around the arm of his desk chair. He pulled himself slowly,  _painfully,_  to an almost sitting position.  _Not yet._ In a few days, maybe a week, they would be able to begin Determination testing with monster subjects, and then…

That dissolving feeling…

He couldn’t  _wait_  a few days.

He was practically snarling as he forced his body upward. He planted one shaking foot beneath him; then the other. He stood, shaking, leaning heavily on the edge of his desk.

No more hesitating. He was out of time. The elevator…he just had to get to the elevator.

He managed to press one hand against the wall and took a step. Then another. The walk to his office door suddenly felt like  _miles._ The elevator would be another thirty yards or so, with no walls to lean on. He gritted his teeth.

Somehow he reached the door and managed to push it open. He rested against the door frame, breathing hard.

**“He…llo?”**

Even his voice was weak. It didn’t matter. The lights throughout the laboratory were all off. Everyone had gone home, even Sans and Dr. Betas. He was alone. He had to do this alone.

He could see the elevator from the doorway. It might as well have been on the other side of the Underground.

**“Do it. Walk. You cannot die yet.”**

He walked, every step slow, every step shaking, every step sending another stab of pain through his whole being. The hallway slid in and out of focus. Halfway there his body gave out again and he went down on one knee. No,  _no._ He would not allow this. He simply wouldn’t.

He pulled himself back up. And there was the elevator.

Just down a few flights and then into the basement laboratory. It wasn’t far.

The elevator dinged and Gaster fell inside, hitting the floor hard. He lay on his back on the elevator floor. He raised a trembling hand and summoned a single hand-shaped bullet, then fired it at the floor button. The button shattered, but the elevator dinged again and its door slid closed.

The vibration of the descending elevator felt like it was trying to shake him into dust.

**“Just a little longer…”**

The door opened to the basement laboratory and Gaster dragged himself out. He sat back against the wall for a moment to catch his breath. The Determination Extractor was in view. So close.

**“Now. Stand. Walk. Finish it.”**

He used the wall to pull himself back up. And that was when one of his arms simply fell into dust.

There was no pain. Just shock. He fell back against the wall, eyes wide, breathing fast. His vision went black for a moment. When his sight returned, all the color had gone from the world. The laboratory was cast in black and white and gray, and there was monster dust on the floor.  _His_  dust.

**“N…No. Not yet, please.”**

He looked at the Determination Extractor, the dark skull hanging from the ceiling, shadowing everything beneath it.

He wasn’t going to make it.

Part of his face began to dissolve.

_No._

With the very last of his energy he  _ran._ The syringes were in a small refrigerator next to the Extractor. He knocked the entire thing over in his struggle to open it, sending syringes and beakers and test tubes everywhere. He grabbed the first one he could. One of his legs turned to dust and he collapsed amidst the broken glass.

The rest of him was dissolving now. He jammed the syringe into whatever was left.

His vision went black.

_Now we see._

For a moment there was nothing but the feeling of dissolving away into nothing. Sinking into darkness.

Then the sensation stopped.

The universe seemed to rupture. Energy rushed through him, sparking and burning, searing its way into his soul. The pain was worse than dissolving. It felt as though he was  _melting,_ shaping into something else, like a spent candle. The sheer power was  _immense,_ infusing his every atom.

And suddenly, living was as easy as  _deciding_  to. Suddenly, returning to a familiar shape was as easy as molding wax. It was all so  _easy._

He opened his eyes and saw his own hands. Two of them. They were trembling but they were  _his._  Colors looked strange, but they had come back.

He patted himself down. His body was still there, whole. He touched his face. There was what felt like a scar or a crack running from the top of his head through his right eye. He closed his left eye; immediately color vanished again, returning to gray. Permanent effects, it seemed. His throat hurt. Had he been screaming?

He looked at his hands again. Flexed his fingers. Experimentally, he summoned a few bullets. They were different now; they were  _sparkling._

He tossed the empty syringe away and stood easily. Energy was still flickering through him like lightning. He felt powerful. It was as if the universe itself was flowing into his soul. He  _understood_  now. He understood  _everything._ Determination was the key to it all. And now he was alive, whole, and _Determined._

His face broke into a grin and then he was laughing. He laughed and laughed, so hard he had to lean against the wall again. He laughed for what felt like hours.

 Then abruptly he stopped laughing. He looked around the laboratory. What a mess. There was broken glass and spilled fluids everywhere, as well as a trail of monster dust leading from the elevator. Seeing the dust made him grin again. The world had tried to take his life and he had simply said  _no._

Still, he couldn’t stop now. No, there was too much work to do! He had to compile his findings, run some tests, take measurements. And of course he had to clean up this mess. It wouldn’t do for the staff to come in tomorrow morning and find monster dust everywhere. No, no, he couldn’t have people asking questions. He touched the odd crack in his head again. Yes, he would need some kind of story, some kind of explanation. Something believable. It wouldn’t go over well if people found out that the famous Dr. W.D. Gaster had injected himself with an untested, potentially lethal substance.

He imagined the look on Sans’s face and laughed again.


	5. For Science

**Summary for the Chapter:**

> Gaster has a plan to strengthen Sans's soul.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

“L-L-Listen, Doctor, a-all I’m saying is there’s nothing wrong with taking some time off!” Dr. Betas was practically flailing. Gaster was almost surprised that he hadn’t started hyperventilating yet.

“You just survived a terrible attack. Y-You’re still injured! You should be at home resting. What if that madmonster comes back?”

Dr. Betas and three other scientists had all but cornered Gaster in his office. The Royal Guards had just finished investigating the crime scene and had finally left. They hadn’t managed to find any evidence of who the “attacker” was or where they had gone. Everyone seemed much more concerned that Gaster was alive, and that the criminal hadn’t succeeded in stealing the human souls. Clearly, that was what they had broken into the lab for, after all.

Gaster gave all of the scientists a warm smile despite his annoyance.

**“I assure you all, I’m just fine. Just a little bit rattled.”** He delicately touched the crack in his face as if it was an embarrassment.  **“I suppose they really did a number on me, but it barely hurts anymore. This attack just proves that we must work harder and faster. We must do our best to give the entire Underground the power of the human soul.”**

“Sir, you know we’re all just worried about you,” said one of the other scientists. What was his name again?

**“I do appreciate that.”** Gaster stood and crossed the room to the gaggle of concerned scientists.  **“But I don’t intend to let myself be intimidated. I have far too much work to do. I couldn’t possibly take the day off. I promise, I will go home early tonight. Would that be satisfactory?”**

He laid one hand on Dr. Betas’s shoulder and the other on the shoulder of the one whose name Gaster had forgotten. They all looked at each other with expressions of varying doubt and worry.

“I…suppose that’s all we can ask for.”

“As long as you promise.”

“A-And get plenty of rest tonight!”

**“Of course, of course,”**  Gaster said and started steering them toward his office door.  **“Now please. Thank you for your concern, but I do have a lot of work.”**

“Oh, s-sure…”

“Er, yeah, we’ll let you get back to—”

Gaster closed the door behind them, then slumped against the door with an exasperated sigh. At least that was taken care of. Now he could finish writing down his findings in peace.

He took a seat at his desk again and unlocked one of the drawers, pulling out a notebook. These reports would be entirely for his benefit. He couldn’t risk someone else learning about what he had done. The entire Underground would then be clamoring for access to Determination. Other monsters might not react to it as well as Gaster had. There were some lingering side-effects that were…concerning. Every once in awhile the energy constantly rushing through him would spike, to the point of being almost difficult to control. His head ached on and off. His color vision hadn’t fully come back, and he was starting to think that it wasn’t going to. Occasionally his thoughts ran away from him and scrambled themselves. The physical effects were one thing, but Gaster was more worried about the psychological effects. He needed to keep a careful watch for any odd changes in his behaviors.

Determination was a powerful thing. He could compare it to what he imagined absorbing a human soul must be like, but absorbing a human soul didn’t seem to have the same instability. Not that he had much evidence for that—no one but Asgore and Toriel had actually witnessed Asriel’s transformation, departure and return.

Perhaps it would be best to stop the Determination experiments altogether, or at least until Gaster was sure of the nature and extent of the side effects.

There was a knock at his door. Gaster stifled a grumble. He slid the notebook back into the drawer and locked it.

**“Come in.”**

The door opened. It was Sans. Gaster couldn’t help a relieved smile. Of all the people he had to talk to right now, Sans was the most tolerable.

“Hey, Doc. Sorry, am I interrupting?”

**“Not at all, Sans.”**

“I would have come by sooner, but I was double- and triple-checking downstairs. There’s a lot of crap that the criminal could have gotten at, but it looks like everything’s fine, apart from what the guards already found. Man, it’s a good thing they ran into you. We’d be pretty  _boned_  if some random jerk had gotten one of those souls.”

Gaster chuckled.  **“Indeed. I believe my presence must have spooked them. I suppose that I…scared them soulless?”**

Sans blinked in amused surprise. “Holy crap. Did you just make a joke? Wow. I’m impressed. You sure you’re feeling okay? You sure you didn’t get hit on the head too hard? Not that you would take time off even if you had.”

Gaster propped his chin on a hand and tried to keep from outright grinning.

**“I am glad that you know me well enough not to ask me to.”**

Sans shrugged. “I trust you. Just don’t expect me to catch you if you suddenly drop.” He paused. “Joking aside…I’m glad you’re alright. Where would the Underground be without the good Dr. Gaster?”

Gaster’s smile flickered just slightly.

**“Where indeed…”**

“I gotta get back to my research, but you think you’ll be up for Grillby’s later?”

**“I could certainly use a drink after today’s goings on. I will see you then.”**

“Sure thing,” Sans said and turned to leave. He stopped with his hand on the door handle. “It’s kinda strange, right? That whoever broke in made it down to the basement lab. I mean, half the staff doesn’t even know about the basement. And the public sure doesn’t. How’d they know it even existed? And how’d they know the souls were down there?”

Gaster stared at the back of Sans’s skull. Sans didn’t turn back to face him.

**“Strange,”**  he said neutrally.  **“Perhaps they had inside information.”**

“Perhaps.” Sans shrugged as if it didn’t matter. “Ah well. I’m sure the guards will catch them. See ya later, Doc.”

With that he was gone. Gaster reached up to rub the side of his head. The headache had come back.

 

***

 

Gaster made up a few legitimate-sounding reasons and the Determination experiments were discontinued. The Determination Extractor was moved to a storage room and the test mice moved to a separate lab near the rest of the test animals. There was no sense in getting rid of them entirely, as they could still yield some interesting data. Gaster wanted to keep an eye on them personally…just in case. A small handful of them had shown marked increases in intelligence and problem-solving skills ever since the experiments began.

Gaster, Sans and Dr. Betas turned their attentions to the time machine instead. The math involved was sound, and Dr. Betas was confident he could engineer and construct the thing, but creating a power source and a safe way to channel soul energy was presenting some interesting problems. It was nice, Gaster thought, to be able to work on all of this without desperation and the looming specter of death constantly hovering over him. The work became an entertaining challenge instead of a source of frustration.

While the side-effects of Determination continued to plague him, the good vastly outweighed the bad. He had more energy. He felt more alive than he had in…years? Decades? He couldn’t remember how long. He smiled more, laughed more easily. And most importantly of all, he felt  _powerful._ Gaster had never been a weak monster, but he had always relied more on science and his own intelligence more than his magic. Now magic flowed through him as easily as water flowed through the channels of Waterfall. Now summoning a storm of bullets was virtually effortless.

With power like this, Save and Reset finally seemed within reach. Perhaps he would even be able to create a Save point on his own, without needing the help of the souls. When no one else was around, he tested himself and his limits. Determination would unlock Save and Reset; he just needed to figure out how.

But after weeks and then months of testing himself, nothing ever happened. There was something fundamental he was missing about the nature of Save and Reset. Something that the sheer, raw power of Determination couldn’t affect.

Instead of letting himself become frustrated, Gaster turned his energy toward a different project.

“I’m not…sure I follow,” Sans was saying. They were in the basement lab late one night, both at separate computers. Sans had been neck deep in calculus equations for the better part of the day and was looking more exhausted than usual. Right now he was half-turned in his computer chair, one arm draped over the back, frowning in confusion at Gaster.

**“I believe I spoke rather clearly, Sans,”**  Gaster said with a mild smile.  **“I want to give you power.”**

“Yeah, I got that much. I’m just kinda lost on the how and why. And the…what you even  _mean_  by that.”

Gaster turned back to his computer and brought up a series of images, charts and fluctuating graphs.

**“I have been thinking a lot lately about your predicament. So I started running some tests and simulations. I believe that it is entirely possible to safely grant you power. Power enough to have full access to your magic and to at least increase your defenses. To finally level the playing field for you.”**

Gaster looked over at Sans, but the skeleton didn’t look as excited as Gaster had hoped. Mostly he still just looked confused.

**“You don’t seem happy.”**

Sans blinked his eyesockets. “Oh, I mean…I’m flattered, Doc. It’s just kind of sudden. I thought we were done with the Determination experiments?”

**“This would not be achieved through Determination.”** Gaster tapped on the screen, pulling up a graphic of a simulated monster soul that seemed to be throbbing with power.  **“We would be utilizing the raw power of the human souls instead. Your ideas for powering our little time machine project were what gave me the idea.”**

Sans got to his feet and went to Gaster’s desk so that he could see the screen better. He stood just behind Gaster’s chair while Gaster tapped through the simulations.

**“Human and monster souls have always been compatible with each other. If they were not, then absorbing a human soul would never have been a possibility. Souls are filled with many kinds of things, but primarily they are filled with energy. If the souls themselves are compatible, then it obviously stands to reason that the energy contained within those souls is also compatible. It is a simple matter of converting energy from the human souls into magic, and using that magic to directly augment a monster’s soul—in this case, your soul. It is the same general idea as a monster using human soul power to channel colored attacks, all without needing to absorb the soul. With only one human soul, this would likely not be possible without damaging it. But with two human souls? It would be, as they say, a cakewalk.”**

Sans studied the graphs and simulations. Gaster pulled up a long series of equations for him to read. Let Sans check all the work he wanted—Gaster was certain that he hadn’t forgotten anything or made any errors.

“This is…fascinating,” Sans said after awhile. “And it really looks legit. But…why bother? There’s always the chance this could harm the souls, right? Not to mention my own soul.”

**“The chances of that are very, very slim. I would never put your soul at risk, Sans.”**

“Yeah. It’s not that I’m worried about so much—I mean, your work is as flawless as ever here.” Sans stuffed his hands in his pockets. “I guess I just…don’t know why you would bother. I know we talked about this before, but I kinda thought you weren’t being serious. Heh. Not that you’re ever anything but serious, right?”

**“Why bother?”** Gaster rotated his chair so he could look Sans full in the face. **“You are my friend, Sans. I want you to be safe. What if it had been you here the night of the break-in?”**

Sans smirked and shrugged. “I’m really good at dodging.”

**“And if their bullet pattern had been too complex? I you had been caught off guard? If you made a misstep and did not dodge in time? If…it had been a human, with true intent to kill?”**

Sans looked away. “Then I guess I would have been pretty boned. Heh. But I’ve lived like this my whole life, Doc. I kind of had to make peace with the idea of dying a long time ago.”

Gaster couldn’t understand why Sans was making this so difficult. Didn’t he want to feel powerful? Didn’t he want to feel even a fraction of what Gaster felt?

Perhaps a bit of guilt would grease the wheels. ****

**“I have not made peace with that idea, Sans. I don’t know how much longer I have left. When I am gone, you will be the one to continue the work on Save and Reset. It will fall to you to find a way to free monsters from the Underground. Who will I count on if something happens to you?”**

Sans rubbed the back of his skull, looking almost sheepish. “Jeez, Doc…”

Time to pull back a little. Gaster sighed.  **“I am sorry. Forgive me for sounding so dramatic. I understand your trepidation, and I will not push you any further. If you do not wish to do it, then…”**

“Hey. I never said I…didn’t  _want_  to. It’s just…you know.” Sans gave a rather shaky grin and spread his arms. “You resign yourself to the idea of living a certain way forever, and then someone offers you a way out, and…it just seems too good to be true, yeah? Like it couldn’t be that easy. You know how many doctors tried to ‘fix’ me when I was a kid?”

Gaster nodded sincerely.  **“I understand that, Sans. Of course there will be risks, and there is always the chance that it wouldn’t work. I would not want to get your hopes up only to have it fail.”**

“No. I…yeah. Yeah, let’s do it. Let’s give it a try.”

**“…Are you certain?”**

“Yeah.” Sans’s grin looked more genuine now. “Yeah, I am. What’s the worst that could happen? Heh. Remind me to never say that ever again. Anyway. If nothing else, the potential scientific breakthroughs are worth any risk. So…let’s do it.”

Sans winked. “For  _science.”_

Gaster beamed.  **“Excellent. I am glad of your willingness, Sans. I believe I can have the soul conversion system up in a matter of days. I merely need to run a few last-minute simulations. One can never be too careful.”**

He turned back to his computer and started typing. There was no time to waste.

“A-A few days, huh? Haha. Wow. That fast, huh. Guess I’d better get myself ready.”

**“I believe in you, Sans.”**

There was a pause before Sans spoke again.

“Hey, Doc? Can you…promise me one thing?”

**“What is it?”** Gaster asked, distracted with building the new simulation.

“Can you promise that this…whole thing, that altering my soul and gaining magic, that…that it won’t change me? I mean…won’t change who I am.”

**“Of course, Sans. I promise.”**

***

 

Sans was visibly nervous when he walked into the lab that night. Gaster and Dr. Betas were already waiting, having borrowed an X-ray room in the medical wing. They weren’t exactly taking X-rays, but the room had all the equipment they needed. The rest of the lab was empty.

Gaster smiled brightly when Sans knocked on the door.

**“You’re later than usual, Sans. Even for you.”**

“Heh. Papyrus was a bit fussier than usual tonight.” Sans tried to look nonchalant and slipped out of his lab coat. “I wanted to make sure he got a really good bedtime story. Just…just in case I’m not back home till late, or…something.”

**“The process should not take more than a few minutes, and any side-effects will be quite minimal,”**  Gaster said as he started running the necessary programs on a computer.  **“And I can all but guarantee you that nothing will go so wrong as to…prevent you from returning to your brother with all promptness.”**

Gaster had been over the simulations and tests and projections a thousand times at this point. He supposed it made sense for Sans to still be nervous in spite of that. His life would change forever after tonight. Gaster just wished his friend could be more  _excited_  at the prospect. It wasn’t like Gaster was going to let him die, or even come to harm.

“Y-You don’t have to go through with this if you don’t want to, Sans,” Dr. Betas said, taking Sans’s lab coat and draping over the back of a chair. “It’s got to be scary.”

Sans waved a hand. “No way. How could I pass up a chance to be part of something this big? I’d have to be a real  _bonehead_  to turn back now.”

Dr. Betas laughed and Gaster chuckled. Sans grinned at them both, but Gaster could see that he was sweating.

“I’d, uh…be lying if I said I wasn’t a little. You know. Nervous. But only a little. So, uh. Let’s get started, right? What should I do?”

Gaster laid what he hoped was a comforting hand at Sans’s back and steered him toward the chair in the center of the room. Sans was trembling slightly.

**“Go ahead and take a seat. We already have everything set up. All we have to wait for is the machine to power up and the program to run.”**

He gestured at the softly humming X-ray machine. It was built into a tube-like structure not unlike the barrel of a gun, which extended like an arm from the wall. Dr. Betas had spent the last few days repurposing the thing. It could never be used for actual X-rays again, but oh well. The two human souls, still in their containers, had been slotted into the machine like batteries.

Sans climbed into the seat and stared at the tube.

“Nice design, Dr. Betas. Doesn’t look freaky at all.”

“Hmph! I d-didn’t make it for its aesthetics, Sans.”

Gaster shifted the machine on its arm so that it was pointed squarely at Sans’s chest.

**“The machine will convert the energy in the human souls into magic. It will then channel that magic directly into your soul. The trick will be keeping the stream of magic constant so as not to overwhelm your soul. From there, your own soul will do the rest of the work, sending the magic where needed.”**

“Yeah, I remember. I’ve been reading over your guys’ notes for days now. So…” Sans gripped the arms of the chair. “Ready whenever you are.”

Gaster flipped a few switches while Dr. Betas typed on the computer. The machine’s hum grew louder and the human souls began to glow brighter. Sans took a few deep breaths.

**“You’ll be just fine, Sans. Though this may feel rather strange. Try to relax.”**

“I’m relaxed. I’m the chillest guy I know. Heck, I should see about moving to Snowdin. That’s how chill I am.”

Dr. Betas hit one last button on the computer, then turned to them.

“R-Ready over here. The computer will regulate the magic flow and automatically shut down if anything g-goes wrong. Which it won’t! D-Don’t worry, Sans.”

“I trust you guys,” Sans said, closing his eyes for a moment.

The machine finished powering up. Gaster’s finger hovered over a large button. He looked over at Sans. The skeleton looked so much smaller than normal in the enormous chair. Sans wasn’t just nervous. He was scared.

Was this really worth it? Just to test what a human soul could accomplish, and what a weak soul like Sans’s could become?

A small voice in Gaster’s mind whispered,  _Yes._

**“Ready?”**

Sans gave him a thumbs up. “Hit me, Doc.”

Gaster pushed the button.

The machine made a  _veeeeeen_  sound and a stream of brilliant white monster magic shot straight into Sans.

Sans’s entire body gave a violent jerk and he inhaled sharply. The lights in his eyes shrank to pinpricks.

**“Sans?”**

“Whoa.  _Whoa._ That’s. Th-That’s  _weird…”_  Sans touched a shaking hand to the spot right below where the beam entered his chest. “Is…Is that what a soul feels like?”

Gaster and Dr. Betas exchanged a glance. It was  _working._

**“So far so good. Just a few more minutes, Sans. You’re doing wonderfully.”**

“A-Always happy to please.” Sans’s voice was shaking. His hand tightened in the fabric of his sweater. “It’s fine. I’m fine. It’s just w…wei…”

Sans trailed off and his head dipped forward a little. He squeezed his eyesockets shut. Gaster frowned.

“Doc…i-is it…supposed to hurt this much?”

Dr. Betas gave a little gasp. “What? It sh-shouldn’t…” He turned back to the computer. “I-It’s running normally. Just like all the s-simulations…”

Gaster examined the machine. It was still operating within safe parameters. Everything seemed normal. That there was pain was surprising, but not unexpected. A soul as weak as Sans’s might not adapt to the new surplus of magic right away.

**“Sans, your soul just needs a few moments to adjust. The pain will pass. Just—”**

“It  _hurts.”_

**“Sans—”**

“D-Doctor, maybe we should shut it down,” Dr. Betas said, wringing his hands “This can’t be right. I-I-I must have miscalculated the—”

**“It’s working exactly as it should. How are his vitals?”**

“They’re within a s-safe range, but—”

“I’m okay. I’m— _ngh._ God, it’s—i-it’s getting worse—”

**“It’ll be okay, Sans, I promise. Just bear with it for a few more minutes, alright? Stopping in the middle might cause damage to you or the souls. Just a few more minutes, Sans, and then it’ll be over.”**

Sans’s breathing had become labored. His whole body was shaking. Wisps of pale energy filtered out from between his bones like smoke.

**“Sans, please try to relax. Just a little longer. Your soul knows what to do; it’s just not used to this much—”**

Sans gave a strangled little whimper and went rigid. The bottom dropped out of Gaster’s stomach. He had never heard a sound like that.

“It hurts, please,  _it hurts,_  it feels like I’m burning,  _please…”_

“Doctor, w-we should—we need to shut it down!”

**“Not yet, not yet. It’s almost over, Sans, just a little longer.”**

“Doctor, i-i-it’s killing him!”

**“It won’t kill him, but shutting it down now almost _certainly_  will! If we don’t keep the stream constant, his soul could shatter or worse! He will survive this. Sans, you’ll be—”**

Sans screamed. Bone-shaped bullets exploded from the wall behind him, destroying several shelves of equipment. Dr. Betas yelped and ducked to avoid the spray of glass.

It was working, but not at all as Gaster had expected or hoped. His heart was pounding. The computer was flashing several warning signs, and the monitor on the side of the machine was glitching. Fifteen seconds left. Gaster reached for a kill switch, hand shaking. Shutting it down now could be catastrophic, but the way Sans was  _screaming…_

**“Almost there, Sans, almost there…Betas, his vitals?”**

“He…I…”

**“Dr. Betas!”**

“E-Everything’s elevated, still in the safe zone, but—”

Sans made a sound like an injured dog and bones erupted from the floor. One struck Dr. Betas and bowled him over. Gaster dodged two that almost went straight into the machine.

**“Six seconds, Sans, we’re almost th—”**

_“Gaster!”_ Sans’s voice broke around his name. “Doc,  _I can’t, please!”_

**“Three…two…one!”**

The machine shut down, the beam of light vanished and Sans slumped in the chair.

Gaster shoved the machine out of the way and summoned his bullets to shatter all the bones in his path. Then he rushed to the side of the chair. Sans was breathing, but his eyelights had flickered out, leaving empty black sockets. He looked like he was in shock.

**“Sans? Sans. It’s over.”** Gaster gingerly touched Sans’s shoulder. The skeleton flinched away as if burned. Gaster withdrew his hand immediately. Nearby, Dr. Betas pulled himself to his feet with a groan.

**“Easy. Easy now. It’s over. You’re alright.”**

The lights came back into Sans’s eyes and he hissed a breath through his teeth.

“I’m…I’m okay…”

“Sans!” Dr. Betas grabbed Sans’s lab coat and ran to the other side of the chair. He carefully laid the coat across Sans like a blanket. “I-I’m so sorry! None of the simulations indicated that…we should have had someone from the medical team here to monitor all this, we should have—”

**“No medical doctor in the Underground would have known what to do about this. Nothing like this has ever been attempted before now. Sans, follow my finger.”** Gaster held up a finger and watched as Sans slowly tracked its movements.  **“Even the most complex of simulations cannot account for everything. There was always the chance of…unexpected effects.”**

Gaster lowered his finger. Sans seemed to be out of the woods.

**“You’re alright now, Sans.”**

Sans sank lower in the chair and covered his face with both hands.

“I didn’t…think it would…hurt that much. I’ve never felt anything like that.”

“How d-do you feel now?”

“Urrrggghhh. Like I took a dip in the magma…” Sans rolled slowly to the side, wincing and shuddering.

“W-Wait, Sans…”

**“Sans, you shouldn’t move yet.”**

“I’m okay…” Sans slid out of the chair, tried to stand and failed. He sank to the floor and curled onto his knees, his lab coat tangled around him.

“Oh jeez, Sans…” Dr. Betas draped the lab coat over his shoulders. Sans pulled the coat tighter, almost vanishing beneath it.

“I’m okay. I’m fine…”

Dr. Betas wrapped an arm around Sans and sank to the floor with him. Gaster looked down at them, one hand tight against his stomach. He felt ill. He had made a mistake. Sans looked so small. Helpless. What had been the point of all this? Why had he thought that trying to strengthen Sans’s soul with something as unpredictable as human soul energy was a good idea? This…this was reckless. Since when was Gaster reckless?

**“Sans, I’m…”**  Gaster crouched down but didn’t trust himself to approach.  **“I’m sorry. If I’d had any idea this would hurt you so much…”**

“Hey.” Sans shifted enough so that he could peer up at Gaster. He smiled weakly. “Hey, don’t sweat it, okay? I’m fine. Everything’s fine. It worked. Check…check this out.”

He held out a hand, palm up. With a flash of faint light, he summoned an array of bones that spun in a wheel above his hand. After a moment he let the bullets fizzle out.

“It worked. Heh. It’s  _easy_  now.” Sans glanced around the room, at the bones protruding from the wall and floor. “Sorry…for the mess, though.”

Gaster smiled a little and laid a hand on Sans’s shoulder. He didn’t flinch away this time. Sans chuckled softly.

“Heh. It was worth it, right?” His eyes closed and he slumped forward. Gaster and Dr. Betas caught him.

“Worth it. I can finally protect him…”

Sans passed out.


	6. You're Blue Now

**Summary for the Chapter:**

> Another human arrives in the Underground, and Sans does something he regrets.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Gaster and Dr. Betas monitored Sans to the point of annoyance over the following week. The skeleton was back on his feet the next day with only some lingering aches and pains, but neither of the other two were taking any chances. After the experiment had gone so…unpleasantly, Gaster was not willing to let anything catch him off guard again. No more surprises.

One or both of them checked on Sans periodically throughout the workdays, and both of them followed him home at night. Gaster had to practically order Sans not to come in for late night shifts. He swiftly got used to a constant stream of “Seriously, I’m fine,” from the exasperated skeleton. Indeed, Sans was showing no signs of side-effects at all, but Gaster knew that such things weren’t always obvious. The side-effects from the Determination certainly weren’t.

Eventually, Sans broke down and told them both that if they were going to follow him all the way home every dang night, they might as well come in for dinner. For the last few days of the week, the three of them ate takeout in Sans’s living room. It was rather nice to have some downtime for once, Gaster thought. This was his first time seeing where Sans actually lived as well. He and his brother shared a small, cozy apartment in New Home.

This was also the first time that Gaster had actually met Sans’s brother. Papyrus was…unexpected. From the way Sans talked about him, Gaster had expected someone quite a bit younger. But Papyrus was only a few years younger than Sans. The two of them were a study in opposites. He looked nothing like Sans, apart from also being a skeleton. He was hyperactive where Sans was virtually sedentary. He was loud where Sans was quiet. While Sans had a brilliant mind, Papyrus seemed…quite a bit less than sharp.

At least he was friendly. The brothers had that in common.

 **“He is…quite the character,”** Gaster said one night after Papyrus had vanished into his own room. Papyrus tended to make himself scarce when any of them said a word that sounded even remotely scientific.

“Yeah,” Sans said, beaming. “Isn’t he cool?”

That was one word for it, Gaster supposed.

“He sure is into the idea of joining the Royal Guard, huh?” Dr. Betas said with a chuckle. Papyrus had said as much at least forty times thus far. “My d-daughter is really enamored with the Guard too. Though, uh, for d-different reasons.”

“It’s all he’s ever wanted to do with his life.” Sans polished off his takeout burger and dusted off his hands. Then he snapped his fingers; the leftover trash lifted gently into the air, surrounded by pale blue light. It sailed effortlessly into the nearest wastebasket. Gaster watched the whole process and kept an eye on Sans, taking mental notes. The magic seemed to come to him effortlessly now. No stress, no faltering.

“He’s always gonna be better with his bullets and magic than me, though, no matter how much I practice this stuff.” Sans grinned. “You should see him. He comes up with these complex patterns, all on his own. He loves patterns and puzzles and such. One time I told him he should go to work in the Core—he’d love all that puzzle building, right? But nope, he wouldn’t even hear of it. It’s always been the Royal Guard for him. Yeah. He’s always had way more direction than me. Always knew exactly what he wanted. Heh. He’s so mad that he has to wait four more years to even be considered for the Guard. Patience isn’t really his thing.”

Gaster wondered if he and his brother had cared about each other as much as Sans and Papyrus clearly did. He couldn’t remember. It had been so long, and Gaster rarely thought about his family. He frowned vaguely as he realized he couldn’t remember what his brother had even looked like.

Strange.

“Aren’t you worried? Being in the Royal Guard can be p-pretty dangerous. Especially with humans coming through every few years.”

Sans shrugged. “Paps can handle himself. He’s strong. Actually, he’s obsessed with catching a human. He’s always out in Waterfall, practicing making traps. Well, that and I think he just really likes those Echo Flowers.”

 **“Sans,”**  Gaster said.  **“Was your desire to be able to protect your brother the entire reason why you agreed to the experiment?”**

Sans froze. Dr. Betas dropped his fork. Gaster blinked at them. He didn’t think he had said anything particularly startling. Sans turned to look toward Papyrus’s bedroom door. It was still closed.

“It—I—that wasn’t…the  _whole_  reason…” Sans rubbed the back of his skull and chuckled a little, trying to play it off. “It wasn’t just—it was for the sake of science, too. To see if it could be done, right?”

Gaster searched him, looking for any trace that it was a lie. Family was of course important, and perhaps it was noble of Sans to risk his health and safety for a loved one. But Gaster had been feeling…different these last few months. Reckless. If he was going to be reckless, then he needed Sans to be the one to keep a level head and a scientific mind. Dr. Betas couldn’t fulfill either of those roles. The lizard monster was brilliant in his own way, but he was always the voice of reason and restraint.

The three of them kept each other in check. The dynamic worked. But not if Sans had a separate, external reason for which he was willing to risk his life.

**“I apologize. That was a personal question. It just…concerns me a little, the idea that you would put yourself at risk for such a personal reason.”**

Sans turned to Gaster with a very strange expression. Gaster tilted his head.

**“What is that look for?”**

Sans opened his mouth as if he was about to say something. Then he glanced to the side and thought against it. He shrugged again.

“Nah, I guess I understand the concern. I get it. You want to know I can stay focused on our work and that I’m not gonna, I dunno, steal the human souls and give ‘em to Papyrus or something.”

**“That certainly hadn’t even crossed my mind.”**

Sans waved a hand. “I know. I’m exaggerating. The fate of the whole Underground is resting on us, and I can’t lose sight of that just…just for one monster. Right?”

Gaster said nothing. Sans turned to look at him again, expression neutral this time.

“Except Doc, you gotta realize something. Papyrus  _is_  the whole Underground to me. He’s my brother. He’s all I have. So yeah. All the work we’re doing, I’m doing it for science and for the good of the Underground, and as far as I’m concerned, that means doing it for him.”

The room was silent for a few moments. Gaster considered Sans thoughtfully.

“W-Well said, Sans,” Dr. Betas said, bobbing his head in agreement. “I think everyone at the lab has s-someone like that. Alphys is the world to me. Since my wife…well, Alphys is all I have left, too. Whenever I’m stuck on something and feel like giving up, I just remind myself that I want my daughter to see the sky someday.”

Sans grinned at him. “Yeah.”

Gaster remained silent.  _Everyone has someone like that._  But Gaster did not. A very, very long time ago, he would have said that it was for his family, but they were gone. Sure, there were a few people he cared strongly for. Asgore, Toriel, Sans, Dr. Betas.

It wasn’t enough. None of them were the reason for why he worked so hard. They were people; people died. If a person was your motivation for something, then what happened when that person died, as they always did? He thought of Sans, Dr. Betas, his entire staff rendered utterly useless by the deaths of their loved ones. Their attachments made them vulnerable.

But perhaps that was Gaster’s constant distance from people that made him believe that. Or perhaps it was the Determination. Or perhaps it was the thought that had been slipping its way into his mind lately: If he successfully Reset this timeline, then everything here, everything from the last several thousand years, would be erased. Not a single thing here—not a single monster—would matter anymore. They would all be gone. Forever.

That had been the goal all along. Since the moment he had seen that last speck of sunlight wink out. But Gaster hadn’t fully realized what that meant.

Until now.

 Gaster looked at his friends.

 **“I suppose it is good to have such a strong reason to keep moving forward, regardless of setbacks,”** he made himself say.

“And it’s pretty amazing of you to want to protect your b-brother, Sans!” Dr. Betas chirped.

“Papyrus can protect himself. But now I can help him out. Heh, he can get in over his head sometimes.”

“Still, I know none of m-my siblings would go through all that for me,” Dr. Betas went on. “I’ve been worried, Sans, b-but you’ve been doing amazing so far. What you went through was h-horrible, so I was sure that—”

“No, wait,  _stop,”_ Sans said abruptly, waving a hand at Dr. Betas to get him to shut up. Dr. Betas clapped his mouth shut with a startled noise.

“Just keep your voice down, yeah?” Sans’s head whipped around to check Papyrus’s door again. “He…he doesn’t know about that part.”

**“What? You…haven’t told him?”**

“Shh. Keep it down.” Sans balled a hand in his sweater, over his chest. “He knows I can use magic now, but he thinks I just—took a pill or something. Haha, you should have seen him when I showed him what I could do, he…he was so happy.”

Sans clutched his sweater tighter.

“He can’t know about—the other part. About what it was like. You guys  _can’t_  tell him about that. Alright? He’d—he’d never forgive me. And honestly, I’d prefer to just forget all about it. The experiment’s over, my magic is stable and I’m doing fine. We’ve got a lot of work to do. You both might as well stop worrying so much about me, so we can move on to other projects.”

Gaster and Dr. Betas exchanged a glance, both of them silent. Sans settled down with a tired sigh and let go of his sweater.

“Sorry,” he said, rubbing at his forehead. “I’m just sick of thinking about it.”

 _Interesting,_  Gaster thought, and then frowned, wondering why he had thought that.

“No, I-I’m sorry,” Dr. Betas said contritely. “Don’t worry. We won’t tell your brother. Heck, I’m fine with never b-bringing it up again.”

**“Perhaps we have been a bit…overbearing of late.”**

Sans chuckled. There. Sans was laughing again. That was much better.

“Yeah, only a bit,” he said, winking at Gaster. “It’s cool though. Been kind of nice having you guys over. We never get to just hang out.”

**“Tomorrow, we will put all this behind us and return to our work concerning the time machine. How does that sound?”**

Sans grinned.

“Sounds  _sanstastic,_  Doc.”

 

***

 

Another human had fallen into the Underground.

It was the same as the last two. They had entered from somewhere near Snowdin, or perhaps on the border of Snowdin and Waterfall. A monster in Snowdin had spotted what they swore was a human, and after that, the human had vanished as if into thin air.

A day later, someone found human footprints and monster dust at the edge of Waterfall.

The discovery had a ripple effect through the Underground. The last time a monster had died at the hands of a human, it had been a guard in a direct confrontation. This time, the human had simply killed a monster and moved on, disappearing into the network of narrow caves and rivers of Waterfall. They were out there, somewhere, and no one knew what their intentions were—or when the human would take another life.

Waterfall was tense. Monsters hid in their homes and the Royal Guard patrolled the caves nonstop.

The mood at the lab was heavy. No one spoke much. Those who lived in or had family in Waterfall were constantly stepping out to make calls. Some of the staff hadn’t even shown up for work. One of those was Sans.

Gaster sat in his office, fidgeting, unable to focus. Sans almost always came to work late, but it had been about two hours now. Gaster tried very hard not to think about what that could mean. It wasn’t like Sans could have been spooked into staying home. There wasn’t very much that Sans was afraid of, it seemed.

At about three hours, Gaster finally got a call.

 **“Sans.”** He couldn’t keep the utter relief from his voice.

“Hey, Doc. Sorry, I meant to call way sooner. Reception’s kinda spotty out here. Heh, they really need to build more towers.”

 **“Sans.”** Gaster felt vaguely like throwing his phone across the room. He pinched the bridge of his nose.  **“Where in the Underground are you?”**

“I’m, uh. In Waterfall.”

Gaster felt his Determination spike. A few hand-shaped bullets crashed into his desk before he got it under control.

**“I beg your pardon?”**

“It’s actually a really funny story,” Sans said thinly. “Ahh, Papyrus, you know how he is—heard there was a human and just, heh, decided to go running out to catch it before, you know, hearing the part about how it killed somebody. Yeah. Heh. Funny. The oversized bonehead is going to bed without a bedtime story tonight, I’ll tell you  _that_  much. Boy do I wish I could still just ground him.”

Gaster drummed his fingers on his desk, ignoring the new crack in the wood. That moronic brother was going to be the death of Sans one of these days, and maybe Gaster as well.

**“Is he safe?”**

“I’m working on finding him. Don’t worry, I know exactly where he’ll be. He set up this trap awhile ago in this one area. I’m pretty close already. Uh, once I’ve sent him home I’ll come into the lab. I’ll even stay late. I’ll get a skele- _ton_  of work done, so don’t worry ab—holy crap, there he is, hold on—”

Sans’s voice faded, but he didn’t hang up.

“Hey, bro. What the heck you think you’re doing out here?”

“SANS! YOU ARE TOO EARLY, BROTHER! I HAVE NOT YET CAPTURED THE HUMAN!”

“Yeah, sorry, buddy, but you’re not capturing any humans today. You’re not capturing anything but a whoooooole lot of chores over this little stunt.”

Gaster heard Papyrus screaming protests in the distance. He rubbed his temples. Sans’s voice faded back in as he tried to talk over his brother.

“God, teenagers, right? Sorry, Doc, I’ll call you back.”

He hung up. Gaster set the phone down with an exasperated groan. He didn’t understand how Sans could be so patient with Papyrus. The more Gaster was forced to interact with Sans’s brother, the less Gaster could tolerate him. Gaster couldn’t imagine a more annoying family member.

A few minutes passed. Gaster was thinking that he should just get back to work and wait for Sans to come in when his phone rang again.

“Hey, Doc. Sorry about all this. I sent him home, now I just need to catch the next ferry. But now there’s a herd of Temmies between me and the waterway, though, so I’m taking a shortcut. I swear those guys can teleport, but never when it’s convenient.”

That, at least, was understandable. A group of Temmies was about as impassable as the barrier itself.

 **“Just get here as soon as possible.”** Teleportation. Hm.  _There_  was an idea. **“And do watch your back.”**

“Sure, sure. How’re things at the lab? Is the Guard keeping us up to date?”

**“Naturally, though there is little to report. Everyone in Waterfall is giving a conflicting story. The last report said that the human had been spotted near the Wishing Room. Though of course, someone else saw it at the marsh, and someone else said that the human had materialized in their kitchen.”**

“Haha. People have some crazy superstitions about those guys.”

**“Quite. To think that at one point I thought that Prophecy nonsense was as bad as it was going to get.”**

Gaster paused, tapping his fingers on the desk again.

**“Since I have you on the phone, I suppose I should at least put your mind to work. I know you so very much regret having to come into work so _very_  late. It would be remiss of me not to utilize an hour or two of unpaid labor.”**

“Jeez, Doc. Such a slave-driver. You’re working me to the  _bone.”_

Gaster pulled out one of his notebooks, flipping to the pages where he had recorded his original findings from the orange soul.

**“Our current situation has gotten me thinking. During the war, one thing we could never understand was why a human who had already killed a monster was so much stronger than a human who had never killed before. A human’s strength would increase exponentially the more they killed. The thought that they could increase their own power by simply killing a few monsters drove some of them…quite bloodthirsty. The seven wizards who cast the barrier spell would have had to slaughter countless monsters to gain the ability to use magic.”**

Sans was quiet on the other end of the phone. No one ever talked about the war, especially not people who had been around to see it, and especially not Gaster. Gaster didn’t have time to feel uncomfortable about the memories coming to him after centuries of dormancy. He was too busy thinking.

“It must have been horrible.”

 **“It stands to reason that such an increase in power would have to be measurable,”** Gaster continued, ignoring the comment. **“There was always an unusual energy signature within the orange soul, though it was barely detectable. The same energy was not present in the cyan soul, but no two souls are alike. At the time, I thought nothing of it. But now I believe that energy reading must have been due to the fact that that human succeeded in killing a monster. Your thoughts?”**

Sans made a few thoughtful noises.

“I remember those readings. It makes sense, I guess. But how does the level increase? You think they’re absorbing monster souls?”

Gaster shook his head, though Sans of course couldn’t see him.

**“No. That is impossible for humans, no matter how strong. For some reason, they can’t absorb souls at all.”**

“Energy can’t be created or destroyed, so it must be coming from somewhere, yeah? So maybe…your regular human, like say the light blue kid—”

**“Cyan.”**

“Eh. Same thing. So the  _cyan_  kid is at like, a sort of baseline. No murdering, no increase in the energy level. But orange kid, well, she kills a poor guard. So she gains something. Something that increases the energy. Or, nah. More like, she gains the energy itself. And if she had killed more, then the energy would have increased.”

**“Interesting. So your theory is that the energy is not present in the average human’s soul?”**

“Can’t say I’ve met enough humans to have an average. Maybe the light blue kid was the one who was unique. I have an idea of some tests to run once I get in. Should be fun.”

**“Assuming you can get yourself in here before everyone goes home.”**

“Heh. Don’t challenge me, Doc. Hey—hold on just a sec.”

Gaster blinked.  **“What is it?”**

Perhaps his brother had returned to yell some more.

“Hold on, something’s making noise—I think a Temmie got separated from rest of the vibrating hordes. Let me just…”

Gaster sighed and waited.

And waited. There were some indistinct noises on the other side of the phone. Then a muffled word that Gaster couldn’t make out. He frowned.

**“Sans?”**

No answer.

**_“Sans?”_ **

“Doc.” Sans’s voice had dropped to a whisper.

**“Sans, what is going on? Is the Temmie trying to buy something off you? Really—”**

“Doc, I…I think I found the human.”

 

***

 

If Sans had a heart, it would likely be pounding.

He was hiding behind a small ledge in one of the narrower caves. Beyond the ledge, the cave opened up a bit to allow some patchy areas of tall grass. The other end of the cave was blocked by one of Waterfall’s many creeks. The human was standing at the edge of the water, picking grass and seeds out of their clothes.

They were unmistakably a human. Sans had never seen the human with the orange soul, so he only had one other specimen to compare this one to, but he had read somewhere that humans were relatively uniform in appearance. The similarities to the little girl with the light blue soul were uncanny. The skin was a bit darker, the hair a bit longer, the clothes obviously different. This one wore some kind of frilly thing around their waist. The shoes were odd. Overall, the human was skeleton-shaped, if you could imagine a skeleton with flesh.

Sans peeked out from behind the ledge, chancing another look. There was a large patch of grass between him and the human, but he could just barely make them out between the blades. They hadn’t seen him yet.

He ducked back behind the ledge, keeping his phone’s speaker covered. Gaster was saying something, but his voice was thankfully muffled by Sans’s carpals. Hopefully the human wouldn’t hear anything over the sound of the water.

A live human. A real live human, right in the middle of Waterfall, and  _Sans_  was the one to find it. He couldn’t believe his bad luck.

He had to stay calm. He was good at staying calm. But he had also never been less than twenty yards away from a live human.

Sans breathed as quietly as he could and lifted the phone again.

**“—ns! Answer me! Sans!”**

“Shh. I’m here.”

**“Oh, thank goodness. What’s going on? What is happening?”**

Sans sank to a crouch and shifted so he could peer out from behind the ledge. The human was just standing there.

“It’s definitely the human,” he whispered, clutching the phone tighter. “It’s in one of the side caves near the main waterway. Damn…I don’t think they saw me. I’m hiding behind a ledge.”

Gaster said something and Sans frowned, unable to translate. It was easy to understand Gaster in person, but it became difficult over the phone or through a computer. It was worse when the doctor became frantic or emotional, which was rare. There was something…visual about the way Gaster spoke that tended to get lost when you had nothing but sound.

“Sorry, Doc, say that again?”

 **“I asked if you knew where exactly in Waterfall you are,”** Gaster said, his voice level again.  **“If it’s near the waterway, then it’s near quite a few monster settlements.”**

A shudder went through Sans. For now there were no other monsters around, but there was always a Temmie or a Woshua passing through this area. Shyren’s pond wasn’t far away. Anyone could show up at any time. Sans couldn’t see a weapon, but maybe this human wouldn’t even need one.

“I’m two caves west of Shyren’s pond and three north of the waterway.” Sans was sweating. “Do you think you can contact the guards and have someone come here?”

The human moved. Sans flinched and ducked lower in the grass, careful not to make a sound. The human sat down on the mossy ground and sighed loud enough for Sans to hear. At this angle he could see the human a little better. It occurred to him that the human was…small. Not as small as the little girl had been, but not very much bigger, either. They wouldn’t be much shorter than Sans.

Another kid?

“What? Sorry, I missed that.”

**“I said I will see what I can do. Stay on the line.”**

“Y-Yeah…”

Sans set the phone on the ground beside him. If the human suddenly spotted him, he would need both hands free. Who knew how long it would take for a guards to get here? In the meantime, he just had to keep his eyesockets on the kid—the human.

The human covered its stomach with a hand.

“I’m s-so hungry…”

The human’s voice startled Sans so much he almost fell backward.

“Why didn’t I save any of that pie? I should have saved some. So dumb…”

They sniffled and rubbed at their face. Was the human…crying?

Sans frowned, watching the human dig into a pocket. They pulled out an ancient-looking cell phone and hunched over it to dial a number. Where in the world had this human gotten a cell phone? Did humans have cell phones? Did some monster give it to them? Did they steal it off of some poor monster’s dust?

The human raised the phone to their ear, still sniffling. Sans leaned forward a little, curious. After several long seconds, though, the human lowered it once more. They clutched it with both hands, staring at it. They looked lost.

“How come you never pick up?” The human sniffled. “Why’d you give me a phone at all if you weren’t gonna answer? Miss…I want to talk to you so bad. It’s just like you said it would be. I-It’s so scary out here. They’re all so scary. Th-these monsters keep attacking me. I d-don’t know what to do.”

The kid was openly crying now. Sans could see the tears streaming down their face. He watched as the kid dialed a number again and raised the phone back to their ear.

“Pick up…please, Miss, please, I need you, please, please pick up…Miss, I’m so scared…you were the only nice one…pick up, pick up…”

No one answered. The kid let out a sob and dropped the phone. Then they drew their knees up to their chest and buried their head in their arms. Sans watched the kid shake with quiet sobs.

There was a muffled sound from his own phone. He picked it up.

“Yeah, I’m…I’m still here…”

**“Good. Any change? Has the human moved?”**

“We’re…still in the same spot. They, uh. They sat down and…tried to call someone on a phone. Now they’re just…crying.”

**“I see. Sans, the Royal Guard is sending someone to your location, but they don’t know when they’ll get there. The Guard is spread thin throughout Waterfall. Poor tactics, really. This is bad. The longer you are there, the more likely the human is to notice your presence. And we cannot afford to let that human disappear again. What’s more, if they reach the waterway…it’s such a heavily trafficked area. So many monsters. So many potential victims. We cannot allow that human to reach the waterway.”**

Sans frowned, watching the kid. They hadn’t moved. They were just sitting there, sobbing helplessly. Like any monster kid who had gotten lost far from home would.

“So what…should we do?”

There was a long silence. Sans heard nothing but the sound of insects in the grass and the child’s crying.

After a minute or two, Gaster finally spoke.

  **“Kill it.”**

Sans felt his eyelights go out.

“…What?”

He must have heard wrong. He must have.

**“Kill it, Sans. You are right there. If you move quickly, you can end it before the human has time to react. Do you have a spare soul container on you?”**

“I…y-yes, but—”

**“Excellent. Good foresight, Sans. Now do it quickly. The first chance you have.”**

Sans’s hands started to shake. He felt his nonexistent insides twist.

“Doc…” He managed to keep his voice level. “That’s not a very funny joke.”

**“I am most certainly not joking, Sans. Don’t be absurd.”**

A breath caught between Sans’s ribs, almost making him choke. He stared through the grass at the child, still on the ground.

“Doc, I can’t.”

**“Of course you can. You have plenty of bullets now at your disposal.”**

Sans lowered the phone and hissed through his teeth, covering his eyesockets with his free hand. That wasn’t the  _point._  Gaster was his friend and colleague, but the doctor could be so  _blind_  to such simple things.

“No, Doc, I mean…I mean I  _c-can’t.”_  His voice shook. He took a breath to get it back under control. “I’ve never killed  _anything_  before. I’m not a fighter. I mean, what the hell, Doc? You can’t just—you can’t just ask me to  _murder_  someone.”

**“It’s not murder, Sans. It’s self-defense! A preemptive strike. That human will kill other monsters or _you_ if you don’t act. Listen…I understand how you must feel, but—”**

“It’s a kid, Doc.” Sans’s voice dropped to the barest of whispers. “They’re just a kid. They’re—they’re a scared, lost kid. What if they didn’t even mean to kill that monster before?”

He heard Gaster sigh through the phone. There was a silence. He had to know that Sans couldn’t do something like this. Gaster had been strange lately, but it wasn’t like he was heartless. Sans could just keep an eye on the kid and intervene if anything happened. Sooner or later a guard would arrive, and then…

And then the guard would kill the kid. Or maybe the kid would kill the guard. They would send more guards, and then sooner or later, the kid would die.

That…that was fine. This had always been the plan. Gather souls, study them, use soul power to break the barrier and escape the Underground. The hope was always that they would discover something about human souls that would allow them to break the barrier without needing the full seven. That was the whole point of the Save and Reset research.

Monsters couldn’t escape the Underground without human souls. One, three, seven. Sans wasn’t stupid. This was the reality.

But they were just a  _kid._

**“Sans…whether they meant to kill a monster or not does not change the fact that it happened. A scared, young human is just as dangerous as an angry adult. You know this as well as I do. We were just discussing the energy levels of a human who had taken monster life, were we not? They will be feeling that energy even now. They will wonder what it will be like to increase that energy. They will become drunk on it. Sans. You _need_  to kill the human. Before that happens. Do you want the deaths of other monsters on your conscience?”**

Sans had to keep himself from yelling. He gritted his teeth hard enough that they creaked. The human was still sitting there. Their sobs had quieted a bit and they were wiping their face on their sleeve. They didn’t look like a killer. They didn’t look bloodthirsty. They just looked sad and alone.

“I don’t want the death of a kid on my conscience, either…”

**“I told you quite awhile ago that our work would take us down some unsavory paths.”**

“I didn’t think that included  _murder!”_

The human lifted their head and turned. Sans froze. The human looked directly at the patch of grass in which he was hiding.

“H…Hello?” the human said in a small voice. “Is someone…there?”

Sans covered his mouth with one hand and the phone with the other, willing himself to stay silent. He could faintly hear Gaster’s voice through the phone. For several long seconds, Sans didn’t even breathe.

The human scooted back a few inches from the grass, still watching it with wide eyes. If they shifted their angle just slightly, they would be able to see Sans through the blades. And then…and then.

Finally, after an eternity, the human turned away. They sighed shakily and rested their forehead on their knees.

Sans breathed again.

He shifted his way backward through the grass, not making even the tiniest sound. He moved back behind the rock ledge, then collapsed against it, shaking.

Gaster was all but yelling through the phone. Sans lifted it back to the side of his head.

**“Sans! Sans, answer me, what’s happening? Sans!”**

“Still here.”

**“Oh…oh, thank god. Sans, please…please don’t scare me like that.”**

Sans tilted his head back until his head was resting against the stone. The ceiling of the cave was lost in darkness. There weren’t even any of the glittering stones here. Just black.

“Doc, I…” He stopped, sighed, began again. “Can’t I just tail the human? Wait for the guard? I can’t…I can’t do this.”

Gaster was quiet for a moment.

 **“And I can’t afford to lose you, Sans. You are my friend, and the best scientist I have. Where would I be if you died at the hands of a human?”** Gaster paused again.  **“I’m running out of time. I need you to outlive me, Sans. I need you to continue my work when I’m gone.”**

Sans almost threw the phone. He hated that line. He  _hated_  it. Gaster had always been manipulative, Sans had known that since day one, and most of the time he was willing to just go along with it—but  _that line_  was the worst. Gaster only used it when he was out of other options. And he had been using it more and more often lately.

It was such a terrible thing to say to someone. And Sans couldn’t even be sure that it was  _true._

“Doc…” He couldn’t keep the tremor out of his voice now. “Gaster, please…”

**“I know it’s not pleasant, Sans. But you need to do this. More monsters will die if you don’t _act.”_**

“They’re just a  _kid…”_

And then suddenly, without warning, Gaster’s voice turned harsh.

  **“I gave you _power,_  Sans. You’re not  _allowed_  to be weak anymore.”**

Something in Sans went cold. He raised a hand to his chest. His fingers curled into his sweater.

Gaster kept going.

**“You need to do this Sans. _Now._  You’ll be protecting the whole Underground. All of us. Think of your  _brother._ What if he is still in Waterfall?”**

Sans…hadn’t known. He hadn’t known that Gaster could be cruel.

On the other side of the ledge, Sans heard the human get to their feet. He peeked around the edge. They were on the move, heading for the opposite side of the cave. Toward the waterway. Toward the rest of the Underground.

**“Sans, are you still there? Sans? Sans, listen to me. Sans you _need_  to listen to me. You look at it and you see a child, but you can’t think of it that way. It’s dangerous, Sans. You don’t understand how… Listen to me. I’ve seen humans, human children, cut down monsters thrice their size. I’ve seen a single child slaughter whole villages. You can’t understand what it’s like.”**

Sans stepped out from behind the ledge. The human was walking slowly, but soon they would reach the exit.

**“Sans? Answer me. _Sans?”_**

Sans dropped the phone and moved. He pushed his way through the grass. The human let out a terrified squeak at the sound and whirled.

“Who’s there?  _Who’s there?!”_

Sans called on his magic. It was so easy these days. No effort. No pain. All thanks to the good doctor.

He was quick. It was over before the kid even spotted him.

Sans stood back, watching a red liquid spread from the fallen human. He covered his face with one hand.

“I’m sorry.”


	7. The Save Point

**Summary for the Chapter:**

> Sans returns with the blue soul in tow. Gaster achieves something incredible.
> 
> Papyrus is in this one.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Gaster paced his office. When he wasn’t pacing his office, he paced through the halls and laboratories, making the staff nervous. Gaster didn’t care.

He was furious with himself. He had lost his temper; he had pushed Sans too hard. When you were trying to convince someone to do something, to do the _right thing,_ you had to know when to pull back. Gaster had  _always_  known when to pull back before now, but he had let his desperation and Determination control him. It had been hours now with no word from Sans and no reports from the Royal Guard. Gaster’s friend, his best scientist, might be a pile of dust in some lost corner of Waterfall, for all he knew. The human could be dead as well, or they could be carving their way through Waterfall right at this moment.

Not knowing something was usually the lifeblood of a scientist, but right now Gaster just felt like it was killing him.

The hours dragged on. Some of the staff began to leave as the workday drew to a close. Plenty of them always stayed late to finish up this or that experiment. Usually Gaster was eager to have the whole lab to himself at night, but right now he didn’t think he could take the silence of the empty halls.

Gaster was in his office going over the notes on the orange soul for about the thousandth time when he heard raised voices out near the front door to the lab.

Something had changed.

He almost knocked his chair over in his scramble for his office door. He burst out into the hall. A handful of scientists had collected in the entryway. Some of them were shouting. One of them cheered. Gaster could just make out what they were saying.

“I can’t believe it!”

“Amazing! We’re safe!”

“How did you do it, Sans?”

And there among them was Sans. Gaster almost sagged with relief. The skeleton was a bit muddy and covered in grass, but he looked no worse for the wear. It was hard to read his expression from this distance.

Tucked under Sans’s arm was a soul container. In it was a shimmering blue human soul.

“Oh, you know,” Sans was saying. “I just got lucky, I guess.”

Sans looked past the gaggle of scientists and finally noticed Gaster.

“But I can tell you guys all about it later. Better hand this off to the Boss, right?”

The scientists laughed, turning to see Gaster. One of them waved at him.

“Doctor, Sans brought the soul! That’s three souls now—we’re almost halfway there!”

Gaster smiled warmly.

**“Indeed we are. This is cause for celebration. Why don’t you all take the rest of the night off? It has been a stressful day. I think this all warrants a round of drinks at Grillby’s. You can tell him to put it on my tab.”**

This caused quite a bit of surprised, excited chatter. Gaster wasn’t exactly known for his generosity or frivolity. But it did the trick. Soon, the scientists had dispersed, leaving Sans and Gaster alone.

Sans hadn’t moved from the entryway.

**“Sans. I can’t tell you how relieved I am that you are alright.”**

Sans walked toward Gaster, shifting the soul container into his hand. Up close, Gaster could now see Sans’s expression. He looked exhausted. Strained.

**“I know that it must have been extremely stressful. But you are alive, and the Underground is safe, thanks to—”**

Sans reached Gaster, held up the soul container and shoved it against Gaster’s chest. Not hard enough to hurt, but hard enough to be startling.

“Here.”

Gaster blinked down at Sans for a moment in surprise. Sans wasn’t meeting his gaze. Gaster took the soul carefully. It was a deep, shimmering blue. Gaster remembered this color. Like an ocean, or like twilight.

**“Sans…”**

He hesitated, studying his friend. Free of the soul, Sans had shoved his hands in his pockets and was staring into the air at some point near Gaster’s chest.

 **“I am sorry…”**  Gaster said slowly, trying to choose the right words.  **“I did not mean to push you so hard. I only—”**

“Gaster.”

Gaster went quiet. Sans glanced up at him, meeting Gaster’s eyes for just a moment.

“Please don’t make me do something like that again.”

Sans’s voice sounded hollow. Gaster stared down at him. The hallway was thankfully empty, and the quiet seemed at once oppressive.

**“I asked you to do the right thing, and you did it. I know I pushed you too hard. I said some things that I regret. But…I did not ‘make you’ do anything, Sans.”**

Sans hunched over a little and let out a short, sharp bark of laughter.

“Hah. Yeah. You’re right. You’re right, Doc. It’s not like you forced me. I could have just let the kid go. Just let them wander off, maybe kill some more monsters, maybe not. Whatever. Whatever, it doesn’t matter.”

Sans moved past Gaster. Gaster caught him by the shoulder before he could get far. Sans pulled out of his grasp.

**“Sans…I think we should talk about this.”**

Sans’s grin was completely mirthless. “I killed a kid, Doc. What else is there to say?”

 **“That is just it, Sans. You did the _right thing._ Child or not, it was a threat to the entire Underground. And one way or another, the human was always going to die.” **Gaster indicated the soul in his hands. **“We needed this soul. One way or another, we were going to get it.”**

He reached out to Sans again, but Sans pulled back.

“Yeah. One way or another. I, uh. I need to go change. I’ve got a lot to catch up on. And we’d better start work on that soul tonight, yeah?”

Gaster frowned, considering Sans. The skeleton started to turn away again, but this time Gaster caught him.

**“Sans—”**

“Let go.”

Sans’s voice was completely neutral when he said it, but it still made a sharp pang run through Gaster. He let go.

**“Perhaps…perhaps you should take the rest of the night off. I am…concerned about you. Perhaps a good night’s rest will serve you well. You’ve had a rough day.”**

That got a reaction from Sans, but not one Gaster had been expecting. The skeleton tensed, frowning.

“No. I want to work.”

**“Really, Sans, I insist. Go home. Be with your brother. Get some sleep.”**

“No.” Sans shook his head.  _“No._  I’m fine. I’m fine, Doc. I need—I need to work, okay? I can’t—Gaster, I can’t just go home and—and see Papyrus and…and go to sleep. I c—I need to work, alright? Can you understand that? Can you understand that much?”

Gaster folded his arms and sighed heavily. He understood. He knew plenty about throwing yourself into work to avoid reality. He also knew how unhealthy it could be.

But he couldn’t force Sans to go home. He had done enough damage already.

**“Very well. Get changed and meet me down in the basement labs in a half an hour. I’ll call Dr. Betas and see if he wishes to join us.”**

Sans closed his eyesockets in relief. Then he nodded, turned, and stalked away down the corridor without another word. Gaster watched him go, then looked down at the blue soul.

It was hard to stay concerned about Sans when he held the third human soul in his hands. Gaster smiled once Sans was out of sight. He had a good feeling about this one.

 

***

 

The blue soul changed everything.

The first night after beginning research, they found something extraordinary: Save power. After all this time, they finally had a human soul who had “saved” at least once in the past. It was lucky that they had captured the soul when they did—otherwise the human might have reset and returned to before their death. Then again, perhaps that had already happened. There was no way of knowing.

They had it at last. Save power. And not just that, but the blue soul provided a new wealth of Determination, as well as what Gaster had tentatively decided to call “kill power.” Like the human with the orange soul, it appeared that this human had only killed one monster, so their power had never increased very much. Still, it proved that the energy associated with killing was easily measurable.

“We’ve gotta come up with a better term than ‘kill power,’” Sans said constantly. “It’s a bit too on the nose.”

Gaster let Sans and Dr. Betas think about that. He was too busy studying all the other things that the soul had provided. The magic technicians had discovered something truly fascinating about the soul; an aspect that would allow for a magic that actually affected the souls of others. It was an extremely powerful magic, not something that the average monster would be able to manage. Monsters, of course, were lining up to try and learn it, but as far as Gaster knew no one was ever successful. The magic was very specific; it applied gravity to the soul of the person it was used against, pinning them to the ground and restricting their movements. A monster would need a specific type of magic to begin with if they were going to have any chance of using it.

Ironically—or perhaps fatefully—one monster with the perfect skillset for learning blue magic was Sans.

 **“Altering the gravity of something is very similar to telekinesis, after all,”**  Gaster explained one night as he flipped through a report on the subject from one of the magic technicians.  **“You are naturally predisposed toward blue magic. It would essentially allow you to move a soul around as you wished, without actually harming the host in the process. A boost to your natural telekinesis.”**

“Hm,” Sans said from where he sat at his computer. He hadn’t really looked up. Gaster watched him for a moment.

**“You could…perhaps think of it as a way to honor the human, if you wanted.”**

Sans was quiet for a minute before answering.

“Maybe. I’ll think about it.”

That was the best Gaster could hope for. He had been very careful with Sans these past few months, trying to give him the space he needed and not be too demanding. It was difficult—there was  _a lot_  that he really needed from Sans, but Gaster was willing to give Sans plenty of time to recover. Sans had gone back to his usual friendly, joking self rather quickly, but underneath the familiar demeanor something was always different. Sans had changed since his “run in” with the human. Sometimes his laughter seemed a bit forced. He worked longer nights, and fell asleep at his desk in the day more often. Other members of the staff had noticed and either expressed their concern or complained about Sans’s lazy attitude. Sans always laughed it off.

Gaster, for his part, had eventually just stopped asking how Sans was doing. The answer was always the same. He was fine. No big deal. He was just tired lately, that was all. Wasn’t he such a lazybones? Gaster settled for just keeping an eye on Sans and trying to make his life a little easier.

In any case, Gaster couldn’t focus on Sans as much as he wanted to. He had other things to think about—namely the blue soul’s Save power. It was finally time to put his Determination to good use.

Every night, after even Sans had gone home, Gaster was still in the basement laboratory, studying Save power and trying to figure out how to use it. Without Reset, a Save was little more than a sort of insurance, but learning to Save was just the first step. Reset would come eventually. Gaster had Determination enough to be patient. In fact, his Determination had been rather…overwhelming of late. It was getting harder to control; sometimes a headache would come on, sudden and sharp, only to disappear a moment later. His bullets would occasionally manifest without him actually summoning them. He had dreams where he was melting or breaking apart.

Trying to Save provided a sorely needed outlet for all that Determination.

The problem was that, over the weeks and then months of trying, one thing became very clear. Saving was going to require an incredible amount of Determination, possibly more than Gaster had access to. Late at night with no one around after hours of trying and failing, it was very easy to let his eyes stray to the storage room where the old Determination Extractor had been moved. It would be easy enough to hook up the Determination Extractor one more time, extract some Determation from the blue soul and simply inject himself again. But Gaster was already dealing rather poorly with the Determination he did have. More Determination might just kill him. There had to be a different option.

Gaster considered magic. A long time ago, Sans had compared both magic and Determination to a “glue” that held humans and monsters together. Judging by his own experiences with Determination, the two were indeed virtually incompatible. But at the same time, they augmented each other. Perhaps he could use extra magic in place of extra Determination. Like substituting something in a cooking recipe.

Monster souls had a limited capacity when it came to magic, however, even Gaster. If he was going to increase his magic he would first need to increase his soul’s capacity.

That meant breaking out Dr. Betas’s repurposed X-ray machine again. Gaster made a few adjustments so that the blue soul could be slotted in next to the orange and cyan souls. He made sure the entire lab was devoid of staff, made his way to the X-ray room and prepared himself for the worst.

There was no pain, however. Not the kind of pain that Gaster had seen in Sans. Perhaps his soul was just that much more powerful, or it was his Determination at work again. It was a  _rush,_  almost exactly like when he had first injected himself with Determination. Magic flooded him and infused his every atom. He could feel it fighting for its place alongside the Determination, and  _that_  was painful, but nothing Gaster couldn’t bear.

When it was over, he felt more powerful than ever. Like he could single-handedly shatter the barrier.

Another crack had appeared in his face, on the left side this time. His vision went…strange. Stranger than before. He studied himself in a mirror after he had safely hidden the X-ray machine again and cleaned up the room. His left eye, the one that still saw color, had acquired a strange glow, like a bluish flame. Turning it off was as simple as thinking about it, but it made his vision odd. It was like he was seeing television static through his left eye, or a glitching computer screen. Things would dart around his field of vision, disappear, reappear, as if he was trying to see multiple images at once. The combined effect with the colorblindness of his right eye was rather nauseating, but Gaster figured that he would get used to it. He had gotten used to the Determination, after all.

Unpleasant side effects, sure, but it didn’t matter. He was stronger. He could use cyan, orange and blue attacks now with no effort at all. His magic felt  _boundless._

Less than an hour later, Gaster created a Save point.

It was such a tiny thing. All those years of hard work and experimentation and sacrifice and there it was, a tiny flickering light on his office floor. Such an oddly simple thing. It was as though he had grabbed a fistful of the fabric of spacetime and nailed it to the floor. The most exciting scientific accomplishment in years.

And yet…

And yet, the longer he stared into the soft glow of the Save, the stranger it began to look. There was something…wrong about it. Gaster couldn’t tell what it was. It was a feeling more than anything. Perhaps his left eye was just making him dizzy.

It didn’t matter. It didn’t matter. Gaster chuckled quietly to himself. It didn’t matter. He would study it and then he would fix it.

He looked up, out his office window, to where the Core glowed in the distance.

Soon now.

 

***

 

The looks on Sans and Dr. Betas’s faces were absolutely priceless.

“I can’t…I c-can’t believe it!”

“How the hell did you pull this off, Doc?”

They were in his office, standing around the Save, staring into the warm glow with unconcealed awe. Sans had crouched down to see it closer. Gaster stood behind them with his arms folded, letting himself be as proud and excited as he truly was.

 **“I have extensive notes,”**  he said, reaching back to pat a thick folder on his desk.  **“You are both free to read through them for the specific details.”**

The notes weren’t even lies. Not entirely. They just left out the parts about what Gaster had done to himself. It wasn’t like he could hide the effects entirely; the new crack in his face hadn’t gone away.

**“Essentially it was a matter of drawing power from all three souls. Creating a Save point, it seems, takes a rather absurd amount of Determination. I daresay that if a single human with that much Determination were to ever come along, they would have the power to overwrite the entire world to suit their whims.”**

“Yeah, no kidding,” Sans said, standing back up and rubbing the back of his skull in amazement.

 **“That amount of Determination combined with the natural Save power of the blue soul were enough to essentially, artificially, use my own magic to generate a Save. With some complex algorithms and even more complex magic, it was…well, not simple. But doable. Though of course…”**  Gaster gingerly touched the new crack in his face. It didn’t hurt, but it felt wrong. Wrong in the same sort of way that the Save did.

**“Of course it took its toll.”**

“I d-didn’t want to ask about that since it seemed impolite…” Dr. Betas admitted, looking up at Gaster with concern. “But are you actually o-okay, Doctor? It reminds me of that time awhile ago that you were attacked…I k-know you didn’t take the day off then, but using that much magic to do something no other monster has done…”

Gaster laughed and clapped his scientists on the shoulders. Dr. Betas squeaked a little in surprise.

**“Oh, I appreciate your concern, Dr. Betas, truly. Even I will admit that I am quite worn out. But how could I take a day off after creating _this?”_**

He gazed down at the Save point. It hurt to look at, but he couldn’t look away.

**“This is one of the greatest accomplishments of my entire scientific career. And I could not possibly have done it without the help of both of you.”**

He let go of them both. It would probably be strange if Doctor W.D. Gaster suddenly hugged his two favorite scientists.

“O-O-Oh! Oh, gosh, well…”

“Heh, Doc, you know we’re only doing our jobs.”

 **“And I appreciate it. Endlessly.”**  He smiled down at them.  **“But our work is certainly not over. This is just a large step forward. A step toward the true goal—Reset. And on that note…don’t we have a time machine to work on?”**

***

 

Sans honestly hadn’t given Gaster’s suggestion a second thought. The problem was that  _Papyrus_ wanted to learn about blue attacks, possibly as much as he wanted to join the Royal Guard.

“It is a chance to STAND OUT! To prove to the whole Underground AND to the Royal Guard just how GREAT I really am! No one else has been able to use blue attacks after all! Blue attacks seem so COOL! A cool power for the COOLEST skeleton!”

“You’re already the coolest skeleton, bro.”

“This is true…but think of how much COOLER I will be with a power like that!”

This had gone on for months now, ever since the science division had revealed the nature of blue magic to the Underground and offered tests and such for monsters to try their luck at learning it. As soon as Papyrus had found out that there was an incredible magic that was not only very hard to use, but also affected the soul, he had been obsessed.

At first, Sans had tried to talk him out of it, and at first, it had worked. For a long while, Sans simply couldn’t handle even discussing the blue soul. Not at home. Working with the soul at the lab was stressful enough as it was. Every time Sans looked at that warm blue glow, he remembered.

He hadn’t told Papyrus what he had done, and of course he never,  _ever_  would. Papyrus had picked up on Sans’s reluctance immediately. Brothers were intuitive like that. For several weeks after, Papyrus hadn’t said a word about the blue soul, and had made a point of never asking Sans how work was going. But as time had gone by and Sans had started sleeping a little better and getting some of his old cheer back, Papyrus had slowly started to bring it up again.

Soon it got to be a daily thing, Papyrus asking every night if maybe  _tomorrow_ could he learn about blue attacks, please? Sans would always answer with a “maybe.”

Finally, though, Sans gave in. You could only handle so much of Papyrus’s sad-skeleton-puppy act before you buckled.

“I’ll be honest though, bro, I’m really not sure how to teach you. I know the theory and the science behind blue magic, but it’s not like I know how to use it. It’s not like light blue attacks, that’s for sure.” Papyrus had taken to light blue attacks like a Woshua to soap. He could be as dense as lead about certain things, but that never meant that he wasn’t clever.

“Clearly you should learn how to use blue magic as well, brother! Then we can learn and fail and try again TOGETHER!”

That was that. And if Sans was honest with himself, it was also the best option. There was only one monster in the Underground who could successfully use blue magic. Gaster had never actually explained  _how_  that was, but Sans didn’t really expect Gaster to explain everything, especially not lately. The problem was that Gaster…well. Gaster didn’t like Papyrus. He never said as much, and he was always polite when he and Papyrus were in the same room, but Sans noticed the look Gaster got when Sans talked about his brother. Papyrus could be a handful, and Gaster certainly wasn’t alone in his completely wrong opinions. But Sans always made an effort to keep Papyrus and the people who didn’t like him as far away from each other as possible.

He didn’t think Gaster would agree to teaching Papyrus, anyway. Not in a thousand years. What he would agree to, though, was teaching Sans. Sans knew that even before he asked.

**“Ah! I had been wondering if you would bring this up. I am glad that you wish to broaden your magical talent, Sans.”**

It was weird, honestly. Gaster was always so enthusiastic about anything that involved Sans “improving” himself. Ever since Sans had told him just how weak he was, years ago now.

Sans had never had the heart to tell Gaster that he was  _still_  that weak. The experiment had given Sans more magic and the ability to finally defend himself, but that was all. Sans himself didn’t mind—he was eternally grateful that the experiment had worked at all—but he had a feeling that Gaster would take it personally.

“Yeah, well, it’s mostly for Papyrus,” Sans said as they finished up for the night in the basement labs. “I’m pretty happy just having magic at all, heh. But Paps is just really into the idea of blue attacks.”

 **“I see.”**  There. That was the look.  **“Still, I would be happy to teach you. Shall we begin right away?”**

Before Sans could even respond, Gaster had stretched out a hand toward him. Something somewhere went  _ding_  and an extremely strange sensation flooded out from behind Sans’s ribcage. It felt like a hand had gripped his soul. There was a heaviness in his ribs that had nothing to do with sadness.

He looked down. There was a blue, heart-shaped glow at his chest.

“Oh…”

 **“Your soul is blue now,”**  Gaster said almost cheerfully.  **“In a battle, your movements would be greatly limited. Perhaps not even you would be able to dodge then.”**

Something about the way he said that made Sans shiver a little. He ignored it and raised a brow ridge at Gaster.

“Doc, I appreciate it, but I was thinking we could, you know, start this tomorrow? It’s late and I’m pretty… _bone tired._  Heh.” He looked down at his chest again. “And can you…ask next time? Or at least give me some warning?”

Gaster blinked at him. He looked genuinely startled.

 **“Oh.”**  Gaster lowered his hand. The glow and the heaviness vanished. Sans patted his own chest, curious. He hadn’t felt his soul so acutely since the experiment.

 **“Goodness. I apologize, Sans.”** Gaster rubbed his chin and frowned at some point on the floor.  **“I was getting ahead of myself. The sensation is quite odd, and rather sudden. I should have asked.”**

Sans tilted his head and gave Gaster a reassuring smile.

“Hey, it’s…it’s fine. Don’t sweat it. It’s not a big deal. Just kinda surprised me is all.”

Gaster didn’t look up at him. It was almost as if the doctor had stopped listening, but Sans didn’t think that was true. Gaster had been so strange lately. Or…not _lately._  For over a year now he had been acting different, and whatever it was, it only seemed to be getting worse.

Sans was constantly trying to reel him back in. Gaster had done some questionable things, especially with regard to Sans, but the doctor was still his friend.

“Hey, Doc, uh. How about a round at Grillby’s? We can talk about blue magic some more. Then we can get started on it tomorrow, in between the real work.”

Gaster looked up at him finally and smiled a little.

**“Yes, I…think I could go for that.”**

The following night, Sans went home early. Papyrus was almost vibrating like a Temmie when Sans got back.

“THERE you are, brother! You are three minutes later than you said you would be!”

Sans gave him a fond smile as he hung up his coat.

“Heh, were you timing me? Sorry to keep you waiting.”

“I can hardly expect you to ever get anywhere on time, you lazybones!!” Papyrus flailed his arms. “But I can forgive you because you are here now and I am VERY excited!”

“Yeah? Ah, you know, I’m so tired, I was thinking of just watching TV for a bit and then going right to bed.”

“SAAAAAANS!”

“Alright, alright.” Sans held up his hands in defeat. “I’m kidding, bro. Just let me put my stuff down. We’d better go outside—don’t want to make a—”

Papyrus was out the door with a cackle before Sans could even finish the sentence.

“—mess. Heh.”

There was a small community park a block from their apartment that would serve nicely. At this hour it was mostly deserted, save for a few monster teens having mock battles. Sans and Papyrus found a nice corner of grass away from the kids.

“So this is kinda complicated stuff. Don’t feel too bad if you don’t get it right away.”

Papyrus stamped his foot impatiently.

“I know, I know! Let’s just get started!!”

“Heh, alright, calm down. So, you’re gonna use your telekinesis to sorta…‘grab’ someone else’s soul. Then you can throw bones at ‘em and make them have to jump to avoid them. It’s pretty sweet.”

Papyrus rubbed his chin and frowned thoughtfully.

“But how do I practice something like that without a human to try it on? Oh! How AMAZING would it be if a human showed up RIGHT NOW?”

He whipped his head around as if a human could materialize at any second. Sans laughed.

“Looks like we’re fresh out of humans, bro,” he said. “But it works fine on monster souls. You can practice on me.”

“Oh…” Papyrus looked like he wasn’t sure what to make of that. “Are you positive it is safe?”

“One hundred percent.”

Papyrus put his hands on his hips and thrust out his chest.

“Do not fear, brother! I, the GREAT PAPYRUS will use caution all the same!”

He always did.

“Well, go ahead, Paps. I’m waiting.”

Papyrus balled his hands and scrunched up his face, concentrating. He looked so funny, thinking so hard. Sans stuffed his hands in his pockets and waited, enjoying the peace and quiet of the balmy night. This was nice. Just practicing magic with his brother, like they’d done when they were kids. No human souls, no time machines, no eerie Save points.

After a few minutes of nothing, Papyrus opened his eyesockets.

“It’s not working!!!”

Sans yawned. “Just think of your telekinesis, bro. Think of using it to grab my soul. It’s gotta be kind of an intuitive thing.”

“HmmmmMMMMMMMM!”

Sans lifted a few inches off the ground. He snorted.

“Not quite.”

Papyrus set him down gently. He didn’t use his telekinesis very often, but he had just as much precise control over it as he did with his bullets.

“This is indeed QUITE difficult. Isn’t there a more straightforward way than just THINKING hard?”

“Sorry, Paps. You know how magic is. You gotta feel it… _in your bones.”_

“ARRRRRGGHHHH!”

Sans chuckled. Papyrus always gave the best reactions to his puns.

“Don’t worry. I said it might take you awhile to get the hang of it. There’s not much magic out there that can affect someone else’s soul.” Sans paused, trying to think of any other tips he could give. “Hm. Try to think of it this way. It’s actually not that hard to affect someone’s soul, right? You can affect someone with how you treat them, the things you say. You affect my soul without even trying.”

Papyrus pressed his hands to his cheekbones.

“AWWWWW! SANS! That is so sweet! That  _is_  supposed to be sweet, right?”

“Haha, I guess. But it’s the truth, yeah? You’re my brother, and you know my soul better than I do. So if affecting my soul isn’t hard and using telekinesis isn’t hard, then, well. You just gotta find a way to combine those two.”

Papyrus had a rare moment of quiet as he considered that. Then he gave a decisive nod.

“I think I understand, brother! I will try again!!”

Sans waited. Papyrus wriggled his fingerbones experimentally, muttering to himself. Then, less than a minute later, Sans heard a now-familiar sound.

_Ding._

He couldn’t help flinching. The sensation was just so  _weird._

“Oh!” Papyrus exclaimed, and the sensation vanished. “Oh, Sans, I’m sorry! Did that hurt you?”

“What, nah, no,” Sans said quickly, waving both hands. “It’s just really sudden when it happens. Hey, you did it, bro! Nicely done. You figured it out so fast. Man. It took me hours.”

Hours of feeling his soul turn blue over and over. You’d think he’d have gotten used to the feeling, but it still felt strange every time. Not exactly unpleasant, either, just…

Sans blinked as he figured it out. Ah. That made sense.

It made him feel vulnerable.

He hadn’t realized how much he had gotten used to  _not_  being vulnerable.

Papyrus was giving Sans a suspicious look, but Sans could tell he was just  _barely_ holding back his glee.

“You are  _certain_  I did not hurt you?”

Sans grinned brightly at him. “You never do, bro.”

Papyrus closed his eyesockets and clenched his fist, mock-solemn.

“Then…that means…”

Annnnd there he went, shooting off like a rocket and running in circles through the park.

“WOWIE! WOWIE! I DID IT! I USED BLUE MAGIC! I CAN USE BLUE ATTACKS! THE GREAT PAPYRUS HAS ACHIEVED HITHERTO UNKNOWN HEIGHTS OF GREATNESS THIS DAY!”

Sans just laughed and watched his brother go.


	8. One Pointless Question

**Summary for the Chapter:**

> Gaster creates a frightening new weapon. Dr. Betas confides in Sans after an accident at the lab. Gaster and Sans have it out, and Sans makes a promise he can't keep.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Gaster could not remember the last time he had gotten a full night’s sleep. Months now, at least. A year, maybe.

His mind kept him awake. There was always some new idea, some new theorem, some algorithm, some computation,  _something_  that his mind wanted to sort through. It was hard enough trying to focus on things these days, and it was that much worse when he tried to sleep. When he did manage to sleep, his dreams were…unpleasant. Not nightmares, not really. They were chaotic, thoughts and images smashing together like particles in an accelerator. None of it ever made any sense.

He thought perhaps that it had something to do with the new properties of his left eye. The flickering images had never gone away; it was like trying to see the world through a glitching screen. Recently, he had been toying with some theories as to what it meant. With access to Determination, impossible amounts of magic and the ability to Save, it was likely that what his eye was seeing was other timelines.

It wasn’t an altogether new theory. Any exploration into time travel and the nature of spacetime itself eventually demanded the question—could other worlds, other timelines, exist? Gaster had always assumed the answer was yes, though of course proving such a thing was difficult if not impossible. Perhaps once the time machine was up and running and they could successfully test Reset, they would have real confirmation. But the machine was only about half finished. For now, Gaster just watched the images his left eye showed him, trying and failing to make sense of it.

Sometimes the things he saw gave him ideas. Between his eye and his unquiet mind, the flood of ideas would occasionally drown out the rest of his thoughts. Someone would be talking to him and would say something that sparked an idea. His mind would wander as he sorted through the possibilities and he would forget that anyone was even talking to him. It was a problem. He couldn’t afford these kinds of distractions; nor could he afford to have his staff thinking that he was…losing himself.

The only way to quiet things down was to pursue some of his ideas, in between research on the souls and work on the time machine. It divided his attention, but it provided at least a modicum of focus. Like extreme multitasking. Sometimes he would let Sans and Dr. Betas in on what he was working on; mostly he kept it to himself.

One idea ended up being something he was quite proud of. Late one night, he brought Sans and Dr. Betas up to the roof of the lab for a demonstration.

 **“Monster magic can always be improved upon,”** he explained to them. **“That is what our magic technicians are working on every day. A monster’s bullets and bullet pattern will generally always remain the same, but what if the individual bullets themselves could be improved on? In a way that does not need to rely on the colored magic that comes from human soul power.”**

It had been startlingly easy to do, simply a matter of redirecting the vast amounts of magic and Determination coursing through him. If nothing else, it was nice to have another outlet for all that power. Sometimes it became so overwhelming that it felt as if it was actively trying to consume him. So he had chosen one of his bullets and improved it.

**“Bullets can always be dodged. Imagine that you could make a bullet so large and powerful that it was virtually unavoidable.”**

He saw Sans and Dr. Betas look at each other thoughtfully.

“Our t-technicians have been trying to pull off something like that for ages, haven’t they?”

Sans shrugged. “Always kinda sounded like overkill to me. I mean, I guess I’m kinda new to it all. But bullet patterns are supposed to be kind of a reflection of the monster. People use them for way more than just fighting.”

Gaster smiled a little.  **“Well. This one would have to be used almost exclusively for fighting. Shall I demonstrate?”**

They already had their notebooks out like good little scientists. He waved at them to take a step back, then lifted one hand. The bullet he had chosen from his arsenal was skull-shaped. That had seemed appropriate.

The thing that appeared in the air above him didn’t even look like a bullet anymore. It was still skull-shaped, but even that had changed. It was  _enormous;_  a gigantic, floating skull that looked like it couldn’t possibly belong to any real monster or animal, some horrifying combination of dog and goat and dragon.

The sound it made when it materialized was unforgettable.

Sans and Dr. Betas stumbled backward away from Gaster and the giant skull. Dr. Betas grabbed Sans and held on for dear life.

 **“I’ve been thinking of what to call it,”**  Gaster said, gazing up at it fondly. **“’Bullet’ just really doesn’t do it justice, don’t you agree?”**

“Bullet? No,” Sans said thinly. “Nope. Doesn’t do it justice. What—how did you even—?”

**“Oh, but you haven’t seen what it can truly do yet. Watch closely.”**

The skull opened its jaws. Its mandible split at the seam. Light gathered in its throat with a sound like a laser powering up. Gaster pointed at a stalactite hanging from a lower part of the cavern ceiling.

The skull fired a blast of energy. The air rippled. The resulting explosion echoed back and forth through the cavern. Magic struck the stalactite and the stalactite vanished into blinding white light. When the light and magic faded, the echo was still roaring like fading thunder. A small cloud of dust drifted down from the cavern ceiling into the magma below.

Gaster turned back to his scientists, grinning. Dr. Betas was shaking and clinging to Sans’s arm. Sans was rigid, his eyelights having shrunk to pinpricks.

The skull winked out of existence.

**“I was thinking something like ‘Gaster Blaster.’ Or is that too cheesy?”**

Sans pulled himself out of Dr. Betas’s grip, absently patting the lizard monster’s hand. He took a step forward.

“It. Uh. Sounds great to me, Doc.” Sans was sweating. “That was…really…”

“S-Something…” Dr. Betas whispered.

“Something,” Sans agreed.

Gaster clapped his hands together.  **“I thought you might like it, Sans.”**

“Y-Yeah. Heh. You know me. Always…down for anything skull-themed…”

“Doctor, if I can—i-if I might be so— _why_ did you—m-make something like that?”

 **“Why?”** What an odd question.  **“The Underground can always use more weapons for the war, can it not? And I found it to be an interesting challenge.”**

Dr. Betas and Sans exchanged another glance. Gaster wasn’t sure that he liked that. It was almost like they were keeping something from him.

“I think we’re just a bit confused,” Sans said. “I know the king calls it a war, but it’s…not, really. Attacking a couple of humans who fall down here every so often doesn’t really count as a war.”

 **“Well, of course.”**  Gaster hadn’t even been thinking about that “war.” He had meant the one before all this. The original war. When he Reset this world, he would need a quick and efficient way to deal with all those human armies of the past, after all.

**“I was thinking of the war to come. After the barrier is broken, it will be as if the old war never really finished.”**

“But a-aren’t we trying to  _prevent_  that?” Dr. Betas whined. Gaster had to keep from rolling his eyes. “I-Isn’t the point of Reset to stop the barrier from ever b-being created in the first place?”

**“Yes, and on the off-chance that we are not fast enough in unlocking Reset, then we must be prepared for what happens after the barrier is destroyed. And what happens after that is _war,_  Dr. Betas.”**

Dr. Betas spread his hands helplessly. “But we’re scientists! W-W-We shouldn’t be fighting! You least of all, Doctor!”

  **“If it happens, _all_  monsterkind will be fighting.” **And they would all die. Every single one of them this time. Even Asgore had to realize that. Monsters would never get the surface back. Not in this timeline.

**“Perhaps it is not a pleasant reality, but it _is_  the reality. Quite frankly, this is something that both of you should be considering as well. A scientist must always consider the possibility of failure. For us, failure means the barrier is eventually broken. It means monsters pouring back out onto the surface, into a world that will not accept them. It means the entire Underground beginning a true war with the humans. And in the event that that happens, monsters will need all the magical weaponry they can possibly have.”**

The two of them looked at each other again. It wasn’t that secret, suspicious look this time; now they both just looked afraid.

Gaster summoned the giant skull one more time to drive his point home. Sans and Dr. Betas both took another step back.

**“Imagine every monster with a power like this. Not even Asgore can use magic this strong, but with some work…we might actually stand a chance.”**

He let the skull vanish. The Gaster Blaster. Yes, he liked that name.

**“Let us just hope that it doesn’t come to that.”**

The three of them were quiet for awhile. Gaster listened to the distant rumble and churn of the magma far below. The Core glowed in the distance.

“Well,” Sans said, trying to break the tension. “I’m sure the magic technicians will be happy to hear you went and did their job for them.”

“R-Right! Right. They’ll be r-really excited to…to, um, try and replicate this.”

Gaster chuckled a little.  **“I’ve already drawn up copies of my research notes. They will find them when they come into work tomorrow morning.”**

“Gonna have their work cut out for them,” Sans said with a low whistle. “But, uh, for the time being—it’s late. We should all probably head out for the night. And, uh. Awesome though giant skull death cannons are, maybe we should…keep the time machine our priority. Yeah?”

 **“Naturally.”**  Gaster waved a dismissive hand.  **“I merely got bored.”**

There was that little  _look_  between them again. What was it? What were they hiding? What were they  _keeping_ from him? He felt a headache begin behind his left eye. Determination prickled through his insides. He could always  _make them_ tell him…

No. No. Where had that thought even come from? He turned away from the pair, folding his hands behind his back and staring out toward the Core. He made himself focus. He forced himself back under control. The Determination and magic slowly died down.

What was happening to him?

Sans and Dr. Betas had been talking. Gaster shook his head, tuning back in.

**“Sorry? What was that?”**

“Nothing. Just—see you tomorrow, Doc.”

**“Oh. Of course. Yes. See you tomorrow.”**

When he looked back a few minutes later, they were gone.

 

***

 

Dr. Betas had brought his daughter to the laboratory for a tour. It was the first time Sans had ever met her. Alphys was a lizard monster like her father, and stood a full head shorter than him Her scales were yellow instead of green and she wore a baggy shirt with some kind of cartoon character on it that Sans didn’t recognize. She had apparently also inherited her father’s nervous disposition.

Dr. Betas had caught Sans on his way in and now the three of them were hovering near the entryway. Sans was actually glad of the distraction. He and Gaster had been hard at work downstairs, and Gaster was in one of his moods. Sans had made some kind of excuse to come upstairs, where there was better light, fresher air and no one shouting hand-shaped bullets at a computer screen because the algorithms weren’t coming out right.

Work had been rather difficult lately.

“She’s planning on interning here n-next semester,” Dr. Betas said with a proud grin, wrapping an arm around Alphys’s shoulders. “Following in her old man’s footsteps! I-Isn’t that great? Alphys, this is Sans.”

Sans grinned at her and offered his hand to shake.

“Oh…!” She shook his hand, blushing and staring at her feet. “I-It’s nice to meet you…my dad’s told me all about you.”

“Likewise. How’s the engineering major going?”

Alphys fidgeted, scratching the back of her head. “It’s…going alright, I guess? I mean…it’s hard, but…some of the classes are fun…”

“Heh, oh man, yeah—does Professor Pogo still teach Differential Equations?”

“Oh…yeah! S-She’s pretty great.”

“Yeah, she was awesome. Though her jokes were pretty…” Sans winked at Alphys. _“Derivative._  Right?”

Alphys covered her mouth and snorted with laughter. Dr. Betas mouthed a  _thank you_  to Sans from behind her. Sans gave him a tiny nod.

“Haha! Oh man, that was pretty bad. My dad mentioned your sense of humor, Dr. Sans.”

“Aheh, just Sans, please. I’m no doctor.” It had been awhile since someone had accidentally called him “doctor.” It made him feel weirdly old, though Alphys wasn’t much younger than him.

“O-Oh, okay. Sans, then. I’m Alphys, but…y-you already knew that…”

“Hey, I don’t mind being told twice,” Sans said with a casual shrug.

Dr. Betas patted his daughter’s shoulder.

“Alph, why don’t you take a look around? I need to t-talk to Sans about some work stuff. You wanted to check out the robotics division, right? They’re straight down that hall. Just don’t bother them too much, o-okay?”

Alphys’s face lit up.

“Okay! Um…it was nice meeting you, Sans.”

Alphys gave a shy wave and hurried off. Sans chuckled a little as he watched her go. She was a lot like her father.

“She’s gonna fit in pretty well around here. Especially if Dr. Elwood sees her going all starry-eyed at his robots.”

Dr. Betas suddenly grabbed Sans’s upper arm rather tightly. Sans blinked at him in surprise. Now that Alphys was out of sight, Dr. Betas’s expression had become nervous.

“Y-Yeah, except…e-except I need to talk her out of interning here,” he said, glancing around the entryway as if afraid someone was listening. “A-And I need you to help me. People listen to you. Where’s, uh, where’s D-Dr. Gaster?”

“Working,” Sans answered, which was code for  _in the basement._ Not everyone knew about the basement labs, and since the break-in a few years ago, neither of them wanted spread its existence around.

Sans frowned at Dr. Betas. “But why the heck wouldn’t you want her working here? This is a great…well.” His frown deepened. “I mean, it’s not a  _bad_  place to intern. We’re the scientific center of the underground. What scientist wouldn’t wanna work here?”

Sure, the place had changed since Sans had joined the team, but it wasn’t like it was a bad place. The people were still good, the mood was still generally positive.

“You’re sure h-he’s…w-working?” Dr. Betas looked past Sans down the main corridor, toward the one elevator that led to the basement.

“Come on, Betas, what’s going on?”

Dr. Betas chewed furiously on his lip for a moment.

“C-Can we talk? In private?”

He looked desperate. Almost terrified. Sans tugged his arm out of Dr. Betas’s grip and patted his shoulder.

“Yeah, man. Let’s, uh…locker room? No one’s there this time of day.”

The locker rooms were down another hallway off the main corridors. It looked like your average gym locker room, except that some of the showers were decontamination showers, and there was a permanent green stain on one wall from a prank gone a bit wrong. The lockers were quiet and deserted. Dr. Betas led Sans back into a corner near the decontamination showers and started pacing, hugging his arms.

“Seriously, Betas. What’s going on? You look like you’re scared out of your mind.”

“B-Because I am!” Dr. Betas paused to look down a row of lockers, as if unconvinced that the place was empty. “I almost wasn’t going to come in today, b-but then Alphys said she wanted to see the l-labs…”

“Hey, okay, come on. Stop pacing.” Sans gripped Dr. Betas’s shoulders. “Now you’re making  _me_  nervous. Come on. Tell me what’s up.”

Dr. Betas stared down at him.

“I-It’s Dr. Gaster. Did he…did he tell you what happened last night? After you left? D-Did you see the mess?”

Something knotted below Sans’s ribs.

“He didn’t say anything…and I didn’t see any mess. Just  _tell_  me, okay?”

Dr. Betas was trembling. Sans let him go. Dr. Betas went back to hugging his arms, but thankfully stood still.

“You remember the mice? The ones we were using in the Determination tests? Well…last night I went to check on them before leaving. I-I usually do. To make sure they have enough f-food and such, right? But when I g-got there, s…some of the mice were…missing.”

Sans cocked his head. Of course he remembered the mice. All of the test mice from the Determination experiments were still alive, all of them having long outlived the average mouse. For the last several years, they had displayed increased intelligence and all manner of unusual behaviors.

“What do you mean missing? Like…they died?”

“No, as in… _missing.”_  Dr. Betas rubbed his temples. “As in, they just weren’t in their cages. They weren’t in the room, they weren’t anywhere. Four of them in total. The four who were always the most different. They’re gone. Th-They escaped.”

“Okay…” Sans tried to process this. “So…there’s four Determined mice running around in the lab somewhere.”

“Honestly, I wouldn’t be surprised i-if they’d escaped the lab entirely. But it’s not really that I’m worried about. It’s n-not like they can spread Determination to monsters. S-So some kind of outbreak isn’t a problem. Th-The problem is…wh-when I told Dr. Gaster…”

Sans waited. He had a feeling he already knew where this was going.

“H-He  _lost it,”_  Dr. Betas said faintly, pressing a hand to his forehead as if he felt dizzy. “He just lost it! He was  _furious._  I’ve n-never seen him so…h-he used to be so calm all the time, level-headed, r-remember? He yelled at me, c-called me negligent. Then he—he summoned that  _thing—_ I don’t even know if he  _meant_  to, he had this surprised look after—h-he summoned it and just  _b-blasted_  the whole lab. H-He killed all the rest of the mice. A-And then he just…stared at me for a second like I wasn’t even there. Th-Then it was like…you know that l-look he gets? When he kind of…he like…he just  _goes away_  for a bit, and then he c-comes back, and he g-gets this look like you just woke him up? A-And then all he said was…‘Go home, Dr. Betas. I should clean up this mess.’”

Dr. Betas fell silent. Sans stared at him for a moment, wishing he could feel shocked or horrified. He felt…tired. Sans slumped back against a locker and dragged both hands down his face with a groan.

 “Dammit,” he muttered.

“I know we keep pretending th-that everything’s fine, but I…I don’t know how much l-longer I can do this. What if he just—goes off on one of us someday? Wh-What if he loses it a-and points that…th-that Gaster Blaster at us?”

Sans shook his head vehemently. “He wouldn’t do that. He’s changed, yeah, but he wouldn’t—he—even on his bad days, he still keeps in control…”

“Y-You know that’s not really true, Sans. E-Especially after last night.”

“…No, yeah. I know.”

It was little things. Like shouting bullets at his computer. Like snapping at one of them for getting a calculation off. Like forgetting what he was saying in the middle of a sentence. Little things, piling up and up over the last few years. It had been slow at first, but ever since creating that eerie, flickering Save point, it had escalated.

Sans remembered the first time he had met Gaster. It seemed almost impossible that the monster then and the one in the basement now was the same person.

“We need to talk to him. He  _knows_  there’s something wrong. He’s too smart not to have noticed.”

“Yeah…but how do you convince someone that smart and th-that important to get help? H-He never even takes sick days. He got attacked and h-had his  _head cracked open_  and he just—stayed at work. Heck…I don’t think he’d ever listen to me. He never listens to me anymore…”

Dr. Betas paused and tilted his head. He gave Sans a searching look.

“He…might listen to you, though. Like I s-said earlier, people listen to you. And you’re his favorite.”

Sans looked away, uncomfortable.

“That’s a weird thing to say…”

“It’s the truth. He always…I-I think you interest him more than the rest of us.”

Sans scratched the back of his neck vertebrae.

“Maybe, but I never did understand why…”

“If anyone can convince him to get help, it’s you. I…I just don’t think I c-can talk to him right now. I r-really don’t know how much more of this I c-can take.”

Sans looked up at him again. “Betas, he’s not just our boss, he’s our  _friend._  We can’t just give up on him.”

“I’m not! I wouldn’t!” Dr. Betas waved his hands. “I w-want to help him. I-I just don’t know  _how._  And…and I can’t. N-Not right now. I’m t-too nervous after last night…”

“Okay, okay, fine. I’ll talk to him.” Sans sighed heavily. “One-on-one. See if he listens. But if he doesn’t, we figure out a time and we talk to him again,  _together_ next time, yeah?”

“Y-Yeah. And i-if that doesn’t work…”

“Then we’ll. Hell. I don’t know.” Sans gave a helpless little laugh. “I don’t know, but we’ll figure something out.”

“Okay.” Dr. Betas took a very deep breath. “I…need to go find my d-daughter.”

“Alright. I’ll…head downstairs. You know…Alphys might still  _like_  working here.”

“Maybe,” Dr. Betas said. His voice took on a rare, hard edge as he turned away. “But until I’m sure it’s safe, I don’t want her anywhere n-near Dr. Gaster.”

There was nothing Sans could say to that. He watched Dr. Betas scuttle away and leaned back against the lockers again, trying to mentally prepare himself.

This was too much all at once. For years now he had been quietly, subtly trying to keep Gaster on course. Aligned. He had thought that he was doing a pretty okay job. Now he had to actually confront Gaster, and Sans had never been good at confrontation. He observed, waited, and said the right thing at the right time. It just wasn’t enough anymore.

He honestly just didn’t think Gaster was even going to listen to him. But he had to try. Gaster was his boss, his colleague, his mentor. His friend.

If there was any chance of pulling Gaster back from whatever path he was on, Sans had to try.

 

***

 

Gaster waited by the elevator, tapping his fingers together patiently, watching the world flicker through his left eye. Sometimes the elevator door was opening, sometimes it was closing, sometimes Sans was already here. Gaster blinked slowly and refocused. Sans was still descending.

Sometimes, like now, focusing was easy. Sometimes he could watch the flickers and glitches and see them just as images, easily discernible from the real world. It was a matter of letting his intelligence and logic take over in place of the Determination. What hadn’t been easy was figuring out how to do it.

He needed his focus for this.

The elevator dinged and the door slid open, revealing Sans. The skeleton did a double-take when he saw Gaster waiting for him.

**“Welcome back, Sans. You were gone awhile.”**

“Were you waiting for me?” Sans blinked up at him. “Sorry. I ran into Dr. Betas and Alphys. He’s giving her the tour. More like  _tour-ture_ if she gets stuck talking to the guys in computer science.”

The door slid shut behind Sans.

  **“Yes indeed. What were you and Dr. Betas talking about?”**

The look of confusion on Sans’s face was almost comical. Gaster smiled just slightly.

“How do you…what do you mean?”

 **“I saw you.”** Gaster tilted his head to one side.  **“Heard you, perhaps.”**

“Weren’t you…down here the whole time?”

**“I see many odd things these days, Sans.”**

There was a silence. Sans slid his hands into the pockets of his lab coat and eased his way past Gaster. Gaster followed him.

“Yeah, well…if I’d known you were trying to listen I woulda spoken louder.”

**“Don’t get cheeky with me, Sans. What were you and Dr. Betas talking about?”**

Sans looked back at him with a grin. “But you know, I’ve got such great cheekbones.”

Gaster reached out and grabbed Sans’s shoulder, making him stop.

**“Sans. What were you and Dr. Betas talking about.”**

Sans turned to him and pulled out of his grip, glaring up at Gaster. “This. This exact thing. You, your temper, your—all of it. It’s gotten out of hand, Doc.”

 **“It’s not polite to talk about someone who is listening,”** Gaster chided.

“Yeah, it’s not polite to eavesdrop, either.”

Gaster narrowed his eyes and folded his arms, peering down at Sans judgmentally. His  _temper._  Of all the absurd things. Gaster didn’t have a  _temper._ Perhaps he had been a bit off-kilter lately, but that was all.

**“You and Dr. Betas have concerns?”**

Sans spread his hands. “That’s putting it mildly, Doc. We’re  _worried_  about you. _I’m_  worried about you. Betas told me what happened last night.”

It took Gaster a moment to remember what Sans was talking about.

**“Ah. Yes, the mice. Yes, thanks to Dr. Betas negligence, there are now several mice running around the Underground, carrying an unstable substance with them.”**

“Yeah, and that’s bad and all. Could even be a bit—hairy.” Sans cracked a faint smile, but he wasn’t even really trying at this point. “But I’m talking about how you reacted.”

**“With justifiable disappointment, I should think.”**

“You literally Gaster-Blasted a whole section of lab.”

**“A momentary lapse in judgment.”**

Sans rubbed at his forehead. “Holy crap, Gaster. ‘A momentary lapse in judgment.’ Holy  _shit._  You really don’t think anything is wrong? You really think nothing has changed about you recently? Come on, man. You’re not an idiot. You _have_ to have noticed. You blow up part of a lab, kill some of the test subjects and then clean it up and pretend nothing is wrong? Nothing seems weird about any of that?”

Gaster’s grip on his arms tightened, fingers clawing against his lab coat.

**“Dr. Betas seems to have exaggerated what actually happened. As he often does.”**

Sans raised a brow ridge. “Then why don’t you tell me about it? Why didn’t you tell me when I came in this morning? Why’d you clean it all up before anyone else could see it?”

Gaster’s focus and control flickered. Absurd. It was all so absurd. Since he had first given himself Determination he had kept a close eye on himself. He had monitored all the side effects, had run tests, had taken thorough notes. Certainly the Determination and then the surplus magic had left permanent traces in his behavior and personality, but it was not nearly as bad as Sans and Dr. Betas seemed to think.

 **“So many pointless _questions.”_  **He took a step toward Sans, forcing the skeleton backward.  **“Dr. Betas told me that some of the mice had escaped. I was understandably upset and _briefly_  lost control of my magic.”**

“Do you even hear yourself? There are no pointless questions when it comes to science. And you’re Gaster. Since when do you ever lose control of anything?”

Gaster bristled and took another step. Sans backed up, and for a split-second Gaster saw nervousness override Sans’s stubborn expression. A very small part of Gaster, the chaotic part that he ignored so well, delighted in that fear.

 **“I don’t lose control,”**  Gaster said quite calmly.  **“Not in the way you are suggesting. Though perhaps I have lost control of _you.”_**

Sans’s eyelights shrank in size. Gaster took another step and Sans came up against a wall.

 _“Control_  of me?” He sounded hurt. “Is that how you see me? You think I’m just some intern who’s gonna fall in line if you bark enough? Nah. You should know me better than that, Doc.”

 **“But don’t you, Sans? You’ve been so loyal until now. Always so eager.”** Gaster grinned down at him.  **“Always willing to do whatever I ask. You even killed a human because I asked you to.”**

The lights vanished from Sans’s eyesockets. He hunched in on himself.

“You…promised not to bring that up anymore…”

**“Don’t misunderstand me. I am grateful for your loyalty. Your friendship. That is why I find this all so _disappointing.”_**

“It’s disappointing that I’m  _worried_  about you?” Sans was no longer looking at him. A voice in the back of Gaster’s mind told him to stop, to pull back. That he’d gone too far. Gaster ignored it.

**“It is disappointing that you have lost faith in me. In our goals.”**

“I haven’t lost faith in anything.  _Damn it,_  Gaster.” Sans looked up at him again, his eyelights flicking back on, anger coming back to his face. “Listen to what you’re saying. Do you realize how crazy this all sounds? I tell you I’m worried, and you—”

 **“Crazy?”**  Gaster stepped closer until he was mere inches from Sans. Sans pressed back against the wall but had nowhere to go.  **“Ah. Is that it, then? You think I’m crazy?”**

“Of course not. Stop derailing. Even  _you_  have to admit that…that something is _wrong_  with you. You’re not yourself. You haven’t been for awhile.  _Everyone_  has noticed that something is wrong. You  _have_  to see it too, Doc.”

Gaster’s mouth curled into a snarl. He was sick of this. They had  _work_  to do. So much work. They were so close. The machine was almost complete, the Core was almost ready. And Gaster was so  _close_  to understanding Reset. Gaster couldn’t look at his Save point these days, but he could always feel it resonating with his soul. The ability to Reset was there, somewhere still buried in his magic and Determination, somewhere in those flickering images he always saw.

He could not lose Sans and Dr. Betas now. He needed them.

**“Perhaps you should drop the subject, Sans.”**

“Not a chance in Hell. You’re going to  _listen_  to me this time.”

**_“Sans._ Do not make me more upset than I already am.”**

“Or what? You gonna Blaster me? Doc, come on. This isn’t you. Something’s _wrong_  with you.”

**“Stop _saying_  that.”**

“Why? Because it’s true, and you know it? Because you’ve been pretending everything’s fine, just like I have? Because—”

_Ding._

Sans jerked and then froze, eyesockets wide as a blue glow appeared at his chest. He stared down at it for a moment in disbelief, then back up at Gaster. And there was the fear. Gaster felt sick at the sight of it, but he couldn’t  _stop._

**“Is this the only way to make you shut up?”**

“Doc—”

Slowly, he reached down and tapped Sans’s chest, right over the glow.

 **“All that power that I gave you,”**  he said in a soft, almost thoughtful tone. **“Did you ever think that maybe I could also take it away?”**

“Doc—Gaster—stop.” Sans was trembling. He’d broken out in a sweat. He laid a hand over the blue glow, as if he was trying to grab it back from Gaster. Gaster simply increased his magic, letting his Determination flare.

 _“Ngh—_ Gaster,  _stop.”_

**“Something is ‘wrong’ with me? What does it even matter, Sans? We’re too close now. We can’t give up. Or are you that much of a coward?”**

Somehow, Sans actually managed to laugh. It was short and sharp, almost mirthless.

“Of  _course_  I’m a coward,” he said around another strained grunt. “Okay, Gaster. One—one more ‘pointless’ question. Shoulda asked this ages ago.”

He looked up at Gaster and all at once the fear was gone.

“Doc…exactly  _how long_  have you been  _Determined?”_

And all at once, Gaster’s mind went quiet.

**“…What?”**

“You  _heard me,”_  Sans ground out. “How  _long_  have you been  _Determined,_ Gaster? Answer me.”

Gaster reeled. His magic abruptly died, releasing Sans’s soul. Sans grabbed his chest again and slumped back against the wall, breathing fast. Not once did his glare leave Gaster’s face.

**“I…that’s…”**

“Since the night of the break-in, right?”

Gaster went rigid.

“I knew it. I  _knew_ it.” Sans made himself stand up straight. “God. You faked that whole thing, didn’t you? I didn’t want to believe it, but somehow I just… _knew._ You gave yourself Determination. No one ever cracked you over the head—the Determination did that. It’s the reason for  _everything,_ isn’t it? You gave yourself an unstable substance that we  _knew_  could have terrible effects on monsters. You lied for  _years._  And now you have me, your friend, up against a wall,  _threatening_ me, and you actually have the gall to keep denying and denying and denying that there’s  _something wrong with you._ Look at yourself!”

Gaster pressed a hand to the side of his head, looking everywhere but at Sans.

**“No, you…you don’t know what you’re talking about…”**

“Please. It’s like when Papyrus would get a cold and then pretend he was fine, like that would make me let him go outside. It’s  _petulant._ God. God  _why_  didn’t I say anything sooner? Why did I let you think you had me convinced? I shoulda…I shoulda said something. Back when you…you never would’ve pushed me so hard to kill that human. You wouldn’t have done all that,  _before._  But you changed. It changed you. And I just ignored it.”

Sans sagged, dropping his gaze and covering his face with a hand.

**“Sans…”**

“I’m sorry, Doc. I shoulda…said something. I just pretended everything was fine. That all the weird stuff you were doing wasn’t a big deal. That the science was all that mattered.”

 **“The science _is_  all that matters,” **Gaster said, squeezing his left eye shut. The images, the flickering, it was too much. His Determination kept spiking, flaring, trying to escape his control. A few of his bullets appeared and then vanished.

“That’s not true.  _You_  matter. You matter to the whole Underground. To the science division, to…me. You’re my  _friend._  I just…I wish I’d been better at it. I could have talked you out of it.”

**“No. You couldn’t have.”**

“I coulda tried.”

**“No.”**

“Why? Why not?”

**“Because I would have died.”**

He had never said it out loud before.

Sans went quiet. Finally, thankfully quiet. Gaster turned away from him, not wanting to look at the skeleton for another second. Stubborn, annoying, well-meaning  _Sans._

**“I was dying. I was turning to dust. I watched myself dissolve. I barely made it to the DTE in time. We had evidence to suggest that Determination might prolong a monster’s life. I had no time. It was the only option. I couldn’t die yet, Sans. There was too much left to do. My entire life, everything I’ve worked for, for thousands of years now…all of it was to save our people. And I was going to die without ever accomplishing that goal. I couldn’t…”**

He looked out across the lab, toward where the time machine stood half-finished in a corner. Toward the three human souls secured to the wall.

**“It was scarcely even a decision. I was desperate. The Determination was my only chance at surviving. But…you are right, Sans. I might have been foolish, but I am not stupid. Nor am I blind. I knew that it would change me. I just…did not know to what extent. I monitored myself. Kept extensive notes. But it seems…it seems that I…”**

Gaster opened his left eye again and looked down at his hands, watching the flickering. He could remember what it felt like to fall apart. To actually  _see_ himself dissolve. This felt almost the same.

He turned to look back at Sans. The skeleton was still against the wall, watching Gaster with an unreadable expression. Gaster couldn’t believe what he had done. What he had said. Sans was supposed to be his friend. If he had let his Determination control him for any longer, he might have…

And last night. What if Dr. Betas had been standing in the path of the Blaster? Would Gaster have cared? Would he have bothered to try and stop?

**“I am…so sorry.”**

“Hey.” Sans took a tentative step forward, then approached slowly. “Hey, listen. It’s not the end of the world. You’re still you, despite everything. And I can help you. You just…need to  _let me,_  yeah?”

 **“I think it is too late to help me, Sans,”** Gaster said, watching his hands flicker. He closed his eye and it stopped. He opened it again and it was like the world was trying to shatter around him.

“Yeah, I don’t buy that.” Sans reached up and took hold of Gaster’s hands, pulling them down until Gaster didn’t have to look at them anymore. Gaster looked at Sans instead.

“You’re still you. You’ve been fighting to stay you this whole time. You haven’t gone crazy.” Sans smirked a little. “You do have a  _crazy_  flair for drama. But hey, between you and my bro, I’m used to that.”

Gaster couldn’t help it. He laughed. Quiet, a little broken, but still a laugh.

“I’ve been trying to keep you in check ever since I noticed something was wrong. But subtly. Now I don’t have to be. I can tell you when you’re going too far. Try and get you back to  _you_  when you start slipping.”

The earnestness in Sans’s voice almost broke Gaster’s heart.

**“And you think that will be enough? You think you can help me? The next time I lose myself, I might hurt you again. I might kill you.”**

Sans actually shrugged. “I’ll stop you.”

As if it was that simple.

Gaster pulled his hands away from Sans’s, gently.

**“And you still would bother, after the things I’ve done to you?”**

“Yeah. It’s a friend thing. I know you didn’t really mean any of it.”

He let himself smile. For a moment, the world felt calm. Solid. Stable. Gaster pretended that it would last.

**“Then alright. I will trust you, Sans. I will listen to you.”**

Sans smiled, and the relief on his face hurt to see.

“That’s all I can really ask for.”

**“Just, please. Don’t tell Dr. Betas about this. Or anyone.”**

“Of course. I get it.”

**“And…Sans. Can you promise me one thing?”**

“Sure, Doc. What is it?”

**“Promise me that…you will know when to quit.”**

Sans frowned. “I don’t…”

**“I know it doesn’t make sense. But soon, it will. Promise me, Sans.”**

“I…yeah.”

**“Say it.”**

“I promise, Gaster.”


	9. Kindness

**Summary for the Chapter:**

> Gaster visits something deep in the Core. Another human comes to the Underground. Sans makes a decision.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

Things were tense for awhile. Gaster’s relationship with Dr. Betas never quite went back to what it had been. It had taken a lot of convincing from Sans just to get Dr. Betas to come back to work on the time machine. Sans was true to his word. He never told Dr. Betas what was actually wrong with Gaster; only that he was helping. Dr. Betas remained suspicious and wary, and became even more so when Alphys began her internship. Alphys was as sharp as her father, perhaps even more so. Gaster was interested in adding another bright mind to the science division, but he only managed to speak to Alphys twice. Both times Dr. Betas was hovering nearby. It was the only time that Gaster could remember Dr. Betas ever having a backbone.

Of course Sans wouldn’t approve of a thought like that, so Gaster kept that one to himself.

Sans had been putting in more effort lately than Gaster had ever seen from him. He arrived on time. He didn’t leave work until Gaster did. He worked hard and barely ever fell asleep at work anymore. It meant that Sans was constantly exhausted, but he never once complained. He was always there. When Gaster felt himself slipping, there was Sans, tugging his sleeve and telling him a bad pun. When his temper broke, there was Sans, talking him down. When reality began to break around him, there was Sans, pulling him back.

Gaster had never been more grateful for the skeleton’s existence. Sans kept him grounded. Gaster was never really able to tell Sans just how  _much_  that meant to him.

But there were times that Gaster hated it. Times that his pride or Determination got the better of him. It made him feel  _feeble,_  needing someone to look after him. Like he was just some old monster, ready to fall apart. The Determined part of him felt downright caged. There was so much he needed to do, so much he  _could_ do, and Sans was just holding him back. And in the back of Gaster’s mind, there was always that little voice, reminding him that very soon, none of this would matter. Soon he would be able to Reset, and this entire timeline and everyone in it would be obliterated.

There was no stopping it now, but Gaster had started to feel conflicted. Erasing the timeline meant erasing Sans. He thought he had made peace with that already. Sans had been his friend for years now, certainly, and the thought of losing him—and Dr. Betas, and his whole science team—had pained him, but was always inevitable. He had told himself that sacrifices had to be made, but now he was having doubts. Gaster had let himself become attached.

Perhaps he would be able to compromise. Maybe bring Sans back with him when he Reset and Save just one thing from this doomed timeline. Sans would be useful in the past, keeping Gaster grounded. If he was going to save any one thing, it might as well be Sans.

The time machine might be able to help with that.

It was almost complete, but it was still presenting problems.

“Using the s-soul power will work,” Dr. Betas was saying one night while they were bouncing ideas off each other. He gestured to some parts of the machine with the pen he was holding.

“I’ve figured out how t-to essentially convert the energy of the human souls into both a fuel source and a catalyst. We hook the souls into the machine once—when it’s f-finished, of course—and it will absorb their Reset power. That will let it move backward to previous S-Saves. There’s just two problems that remain. The f-first…we still don’t have a way to move back  _past_  a Save. Th-The human souls just don’t have that power. I-It’s not a matter of Determination or energy, even. Spacetime j-just doesn’t behave that way.”

 **“Easily solvable.”** Already solved, but Gaster couldn’t tell them that yet.  **“It’s just a matter of correctly bending spacetime itself. Simple science, not magic or soul-related. I already have some ideas.”**

Dr. Betas gave Gaster a dubious look that Gaster had grown very familiar with over the last few weeks.

“A-And I guess you won’t be sharing those ideas?”

**“Not yet.”**

“Not yet, eh?” Sans said from the other side of the room. He was sitting on his own desk, halfway through a fifth cup of coffee.

Gaster looked at him. Sans met his gaze and held it. Gaster sighed.

**“I have some tests I want to run before I make any real announcements.”**

Sans shrugged. “Fair enough.”

“A-Anyway,” Dr. Betas continued, not sounding very mollified. “The other problem is that the soul power we do have just i-isn’t enough.”

**“Not enough?”**

“No. If we tried to activate the machine w-with just the power generated by the three souls, th-the machine would just shake to pieces. And it would shatter the human souls. We can’t go destroying souls. O-On the offchance it doesn’t work, we’ll still need those s-souls to break the barrier. Three souls just isn’t enough. We’re…gonna need a fourth.”

Gaster resisted the urge to slam his fist on his desk, just barely. Sans was watching him.

**“We do not have _time_  to sit around and wait for another human. It has been over a year since the human with the blue soul came to us. There is no way of telling how long it will be. Years, decades.”**

“We got time,” Sans said calmly. “We’ve had to be pretty patient so far. And maybe it won’t be that long. The first two humans weren’t that far apart. Less than a year, yeah?”

Gaster couldn’t help a muted grumble.  **“Only because the second one came here looking for her sister.”**

“O-Oh. The f-first two humans…they were sisters?”

“Wait…what?”

Dr. Betas merely looked surprised. Sans looked…rather thunderstruck. Gaster blinked at them.

**“Yes. Sisters. The orange human was shouting it at the guards when they confronted her. She believed that we monsters had killed her sister.”**

“Oh. Then…th-that’s why she fought back,” Dr. Betas said. “That’s sad.”

“Doc, you didn’t tell me…us, that they were sisters.”

Sans was staring at the human souls faceted into the wall. Gaster watched him for a moment.

**“Does it matter?”**

Sans closed his eyesockets, barely longer than a blink. Then he leaned back on his desk and yawned.

“…Nah. Not really.”

“You know…” Dr. Betas said quietly. “Sometimes I wonder if we’re really—”

He cut himself off and abruptly got to his feet.

“Never mind. A-Anyway, those are the two major obstacles we’re face. The m-machine should be finished in a few weeks and we can run some basic tests, but…w-we’re not gonna be able to manage anything until another human falls down.”

Gaster didn’t think he could wait that long. He didn’t know how much time he had before the Determination finally took him over. Maybe a year. Maybe less. Sans might keep him sane a bit longer, but not forever.

“I think I’m gonna sleep on it,” Dr. Betas went on, gathering up his things. “It’s r-really late. I’ll see you g-guys tomorrow and we can see if maybe there’s some alternative w-we can think of.”

**“Mm. Goodnight, Dr. Betas.”**

“See ya.”

Dr. Betas left. Gaster watched the elevator flicker and skitter back and forth. It was getting worse. Sometimes he could see whole areas of world, hear entire conversations.

Once again, he was running out of time.

“So,” Sans piped up, snapping Gaster back to reality. “Not sure what else we can really do tonight. Think my brain is all stormed out. And I don’t even have one, heh.”

Gaster got to his feet and crossed the lab to the time machine. He laid a hand on the smooth metal of its door and leaned against it, sighing heavily. It was ironic, almost. There was never enough time, and yet time was exactly what they were trying to break through.

“Don’t worry too much, Doc. I got your back.”

**“Yes, but for how much longer?”**

“For as long as this takes.” Sans yawned again. “Might still not know what you meant about ‘quitting,’ but quitting on the machine or on you just ain’t in the cards.”

Gaster glared at the polished metal. He could see his own distorted face staring back at him. Sans had no idea what he was talking about. That was Gaster’s own fault for not explaining, but it was still annoying.

“You still here, Doc?”

**“Yes, Sans, I am still here.”**

“So, uh…why didn’t you tell me about the first two humans being sisters?”

Gaster looked back at him, but Sans was staring at the souls again.

 **“I didn’t think it mattered. They are just humans.”**  He studied Sans for a moment.  **“Are you alright, Sans?”**

Sans met his gaze finally. “Yeah. Guess I’m just thinking about…how far I’d go if Paps went missing and got killed.”

Papyrus. It always came back to Papyrus. Sans’s idiot brother was going to pose a real obstacle to Gaster’s plans for Sans.

“Yeah, I know. You don’t get it.”

**“I didn’t say anything.”**

“You don’t need to.” Sans sighed and leaned back on his desk again, letting his eyes drift closed. “Doc, do you ever…stop and think about what we’re doing? Like really stop and think? Do you ever wonder if we’re…maybe…we’re not really doing the right thing?”

**“No. Never. And neither should you.”**

Sans sat up straight, going back into watchful mode in response to Gaster’s vehemence.

“Whoa, easy. I’m not saying I’m having second thoughts. We’ve gotta get out of the Underground somehow. It’s just…the methods we use.” Sans paused. “And I’ve been wondering. What do you think will happen to this timeline when we Reset?”

Gaster turned away. Sans was too good at reading his face, and Gaster couldn’t risk Sans catching him in a lie. The world flickered and shifted. His Determination spiked.

**“I do not know. I believe that the Reset will create an alternate timeline, one in which the barrier was never formed. This timeline will likely continue on as though nothing has changed, however.”**

“See, but I’ve been looking at some of the projections and—”

 **“The point is, we have no real way of knowing how this timeline will be affected until we are able to run the initial tests. I believe this timeline will remain intact, but there is also the possibility that this timeline will simply…”** Gaster waved a vague hand and accidentally summoned a few of his bullets. He let them vanish before he continued.  **“…will simply refold itself into the new timeline. It should be quite seamless. If spacetime did not have a way of easily reorganizing and repairing itself, I am quite certain that none of us would even be here.”**

Sans was quiet for awhile, flipping through some papers.

“Hm…I guess I’m just concerned with how little we know.”

 **“Such are the risks of scientific endeavors that no one has ever attempted before.”**  He was back under control. He turned to Sans again.  **“But it _is_  the right thing to do. The alternatives are far worse.”**

Sans yawned again and slid off his desk. He looked so tired.

“I guess you’re right, Doc. So what now? Any more tests we could run tonight?”

Gaster sat back down at his own desk and started clicking through tabs on his computer.

 **“I want to go through some of the data again. See if there is something we have missed.”** Gaster’s voice softened.  **“You should…go home, Sans. You’re exhausted.”**

“Yeah, nice try, but you’re not getting rid of me. I’m down for some number crunching.”

**“You really don’t need to babysit me, Sans. I am sure that your brother is waiting for you.”**

“Eh, Papyrus is a big monster. He doesn’t need a bedtime story  _every_  night.”

Gaster’s fingers paused above the keyboard. A large part of him was  _delighted_  to hear that from Sans. If Sans could take a few steps back from Papyrus now, then maybe, when the time came…

But some old part of Gaster rose up inside him and took control before the Determined Gaster could form a response. He turned in his chair.

**“Actually…I think I am, as you would say, ‘all scienced out’ for the night. Dr. Betas was right. We should all sleep on this.”**

Sans gave Gaster a very suspicious look.

“Really? You, Dr. W.D. Gaster, actually  _want_  to be done for the night?” Sans smirked at him. “You sure you’re not trying to trick me into leaving so you can just sneak back here after I’m safely home?”

Sans really did know him too well. ****

 **“Of course not. I am quite serious.”** Gaster stood as if to prove it and put his computer in sleep mode.  **“There just isn’t anything more we can do tonight. We should both go home.”**

Gaster didn’t miss the grateful expression on Sans’s face, though the skeleton tried to hide it. This was right. Kind. He couldn’t remember the last time he had been kind.

He wondered if he would be able to again, after this.

“Alright, Doc. If you insist.”

They left. Sans was clever enough to make sure Gaster was heading in the right direction before departing, and Gaster was clever enough to mislead him. Once Sans was out of sight, Gaster turned back.

But he didn’t return to the labs. He went to the Core.

He made his way through the labyrinthine halls and puzzle rooms, acknowledging the Core workers only when they said hello. The deeper he went, the more the staff thinned out. There were areas of the Core that no one still living even knew existed. Gaster remembered the route, even though the flickering shifts in his left eye tried to confuse him. Down a forgotten flight of stairs to an ancient freight elevator. Deeper and deeper, into the hidden heart of the Core.

At the very bottom was a cavernous room. Down here it was close enough to the magma that there was a constant, purring rumble. The walls glowed faintly orange, despite layers of heat shielding. Here the elevator opened onto a single railed catwalk above…it. The thing that Gaster had hidden in the Core when it had first been constructed. All of the blueprints of the Core that included this room and what it held had long since been destroyed.

The floor was fifty feet below the catwalk. And in the middle of the floor was darkness.

Years ago it had been small, only a fist-sized circle of dark, too small to see from this height. Now it yawned wide, at least thirty feet in diameter. Almost big enough.

It wasn’t black, not really. Looking down at it, watching it flicker in his left eye, Gaster thought it didn’t really look like a color. It was like a hole in the world. Like a void, like absence. It was too dark to see that the thing was moving, but Gaster could feel it, in the same way he could feel his Save point, far away at the lab. It _churned._

He leaned against the guard rail, unpacked a few instruments from his bag, and began running a few tests. Energy readings, oscillation measurements. He dropped a few glowsticks into the hole and watched them vanish, taking notes on everything he observed.

Darker and darker. It was almost ready.

 

***

 

Two months later, a fourth human was spotted in the Underground. This one made it all the way out to Hotland.

This time there was little more than rumors. There had been no murders, at least not obvious ones. No random piles of monster dust found between Snowdin and Waterfall. The human had been spotted only a handful of times, and the rumors painted a picture of someone who was quite friendly. Most people weren’t even really sure that it was a human at all.

It was a trick of some kind; Gaster was sure of it. He could remember a time before the war when humans and monsters had been at peace, when humans could still be kind and compassionate. The war had killed all of that compassion. Humans were generally a mix of good and bad, just as complex as any monster, but there was simply no way that a human had come to the Underground with the goal of making  _friends._

Friendly or not, it didn’t matter. A fourth human soul was the very last piece of the puzzle. The machine was virtually complete. Everything else was ready. One more soul and finally, at long last, they would be able to Reset.

Gaster paced the basement lab. Sans and Dr. Betas watched him warily.

**“The human must be somewhere between Waterfall and Hotland. There aren’t many places in Hotland for it to hide, nor is Hotland a particularly healthy climate for humans. So where could it be? How has it not been captured yet? The guards must be useless. I would be better off tracking down the damn human myself.”**

He had a feeling that he could do it, too. He at least knew what a human looked like, which was more than most of the incompetent guards could say.

“Easy there, Doc.” Sans’s body language was relaxed, but his eyes were tracking Gaster’s every movement. “Someone will find the human. The guards will…deal with it, or they’ll just send them to Asgore and have him sort it out. We just have to wait a bit.”

**“I grow very tired of waiting.”**

“W-We’re all sick of waiting,” Dr. Betas said, fidgeting. “But what can we do? Well, actually…maybe we could post pictures of humans on the television or something? So that m-monsters know what to look for?”

 **“And have a panic like last time?”**  Gaster’s voice was just sharp enough that Dr. Betas flinched a little. Gaster tried to tone it down.  **“It would either be a panic or it would be chaos. Monsters would actively try to hunt down the human, possibly even absorb its soul.”**

“That’s what the Guard is for,” Sans pointed out. “So that the human gets sent to Asgore before that can happen. Monsters aren’t idiots. They know better than to try and take on a human on their own.”

**_“Some_  monsters are idiots. Enough monsters are idiots. If one of those idiots accidentally absorbs the soul, everything is lost.”**

“N-Not really  _everything,_ I’d say…”

Dr. Betas wisely trailed off when Gaster shot a look in his direction.

 **“Suppose the human has deceived someone?”** Gaster paused, gritting his teeth and gripping his chin with a hand.  **“Suppose a monster is sheltering it? I have suspected for awhile that someone has been doing so with previous humans. Perhaps someone near Snowdin. Sans’s description of the third human’s behaviors in Waterfall always struck me as odd.”**

The question was, who would be stupid or naïve enough to do such a thing? To actually  _harbor_  the enemies of all monsters, even if only for awhile?

“I…guess that could make sense.” Sans looked uncomfortable, as he always did when anyone brought up the human with the blue soul. “It seems unlikely that these humans could get so deep into the Underground without someone’s help. We still haven’t found an entrance, right?”

 **“Not yet.”** Gaster went back to pacing. His working theory had always been that the entrance was somewhere back in the Ruins, near where the original cave entrance had been. But the Ruins had been sealed off for hundreds of years now. No one lived there, except perhaps for ghosts, who lived wherever they wanted.

Sans shrugged. “Maybe we can ask the human when they catch it.”

**“If, when, maybe. No, no more.”**

He stopped in front of his two scientists and took a moment to bring himself back under control before he spoke again. He was tense; on edge. He hated being so close to his goals and have them  _still_  be out of reach.

**“No, I have decided. I will go find the human myself. That way we can _guarantee_  that the soul comes straight to us and that no harm befalls it.”**

Dr. Betas ran both hands over his head spines, looking distressed. Sans just kept his eyelights on Gaster. These were the only times that Sans could be described as “alert”—when he was making sure that Gaster wasn’t about to…explode or shatter or attack someone, or whatever it was that Sans thought could happen.

“Doc, you’re a scientist, not a guardsman.”

**“I have fought humans before. And I am likely one of the few people in the Underground other than Asgore who could stand up to a human with a high Level of Violence.”**

They had finally settled on a term for it. Rather crude, but it got the point across, so to speak.

“Okay, let me rephrase,” Sans said, frowning now. “You’re the Royal Scientist. You can’t just go out and kill humans. You’re supposed to do the science that actually gets us out of the Underground. Asgore and the guards deal with the humans. That’s how it’s supposed to work.”

Gaster stepped closer to him so that Sans had to crane his neck and look up. A flash of annoyance crossed Sans’s face.

**“Please don’t outline my job description for me, Sans.”**

“Ease up, Doc. Actually, ease  _down_  a bit.”

“S-Sans has a point, though…” Dr. Betas piped up. “Y-You can’t just go out and…erm…and start k-killing people, even if it’s a human. C-Can’t we just wait for the King? Or one of the guards? No offense, Doctor, but even if you’ve fought humans before, it would have been a v-very long time ago, and human tactics and weaponry have probably changed a-a lot since then. What if the human killed you? You c-can’t risk yourself like that.”

Gaster dragged a hand down his face, frustrated and angry and trying very hard not to blow up at the both of them. He  _knew_  that they understood how important this was…and of course, he also knew that they had valid points. But he most certainly didn’t  _like_  it.

“Dr. Betas is right. You can’t risk yourself. It sounds like this human’s LV isn’t even that high, but it doesn’t matter. We know that even a low-LV human can easily kill a monster.”

**“Not a monster like me.”**

“And see,  _that_  sounds like overconfidence.”

Gaster glared down at Sans and Sans glared right back.

“Th-There aren’t many people in the Underground who have faced a human of any LV and survived,” Dr. Betas said, trying to step between them. “Asgore and th-the guards, that’s all. So…so let’s just let them handle it, okay?”

 **“And Sans,”**  Gaster said, watching Sans’s eyesockets narrow even further. **“Proving that even an untrained monster could kill this human and take their soul. No, I have made up my mind. The risk is too high; too much can go terribly wrong. I will do this myself. I am not concerned about my own life, and even if something were to happen to me, I trust the two of you to continue my work.”**

“D-Dr. Gaster, don’t say that!”

Gaster had already turned away and was heading to his desk to gather his things. He could already see scenarios flashing through his left eye—some random monster absorbing the human’s soul, the human deciding to start its inevitable killing spree in Hotland where all of Gaster’s scientists were stationed, the human falling in the magma and taking its soul with it. Too many variables, too many risks. He  _needed_  that soul, and he was so  _sick_  of waiting and hoping. No more. He would find the human and kill it before everything went wrong and the possibility of a Reset slipped out of his grasp for another year, or another decade, or forever.

He was so focused that he almost didn’t notice that Sans was speaking.

**“What? What did you say?”**

“I said stop. Doc, you can’t do this. I can’t let you.”

Gaster turned very slowly around to face him again.

**“I beg your pardon, Sans?”**

Sans hissed a breath through his teeth, then strode across the room toward Gaster, looking almost as Determined as Gaster felt.

“I said I  _can’t_  let you. I’m supposed to be the one who stops you from doing stupid shit like this.”

Gaster considered just turning Sans’s soul blue again and hurling him across the room. His hands curled into fists. Control, control, control.

Sans kept going.

“The Royal Scientist can’t be someone who goes out and hunts down humans. But more than that,  _you_  can’t go out and risk your life. The human might have no LV at all, or they might have tons of it. You might be more resilient than a lot of monsters, but you can still die. And then where will we be? Even me and Dr. Betas together can’t work that machine. We need you.”

**“Sans—”**

“And you keep mentioning a random monster absorbing the soul,” Sans said, lowering his voice a bit. “But what if that random monster is you? Your self-control is a bit— _out_  of control these days.”

Gaster blinked, some of his anger dissipating. He…hadn’t even thought of that. There had always been that niggling little desire in the back of his mind whenever he looked at the human souls. His Determination practically begged him to just break the containers and absorb them,  _all_  of them. Would he really be able to resist after killing a human with his own two hands?

“So, if it can’t be you, and you’re too impatient to wait for the guards or Asgore to do things the right way, then…well, like you said, there’s one more person who’s faced down a human and survived.” Sans smiled the bitterest smile Gaster had ever seen from him. “If you can even call it ‘facing down.’ No matter how you look at it, I’m more  _expendable_ than you, Doc. Right?”

Gaster felt the bottom drop out of his stomach. He leaned down so that he could grab Sans’s shoulders.

**“Sans, no. You have never been expendable to me. And I couldn’t possibly ask you to do something like this. Not again.”**

Sans pulled out of his grip and took a few steps back. “Yeah, but you’re not asking me. Your hands stay clean this time. Ain’t that a good thing?”

“Sans, w-wait, you can’t!”

Sans had his back to both of them as he walked to his desk.

“Look, don’t worry about it,” he said as dug a spare soul container out of a drawer. “I know a lot of people in the Underground, a lot of people who’d be happy to give me info. I’ll just track down the human and see what happens. Doc says we can’t wait, so we can’t wait. Maybe they’ll be dead already by the time I find ‘em.”

“A-And what if  _you_  end up getting hurt, or w-worse?”

“I’m good at dodging,” Sans said with a casual shrug. He tucked the container into his bag and gathered up his coat.

**“What about your brother?”**

Sans paused in the middle of pulling on his coat. “…What about him?”

**“What happens to Papyrus if the human kills you?”**

“I don’t plan on dying.”

 **“No one ever plans on it, Sans,”**  Gaster said, giving Sans a very pointed look. Sans either didn’t see it or was ignoring Gaster. Probably both.

“I’m just gonna find the human.” Sans finished pulling on his coat. “That’s all I’m _slaying._ Heh. Get it?”

 _“Sans.”_  Dr. Betas crossed the room to him and took hold of Sans’s arm. “C-Can I _talk to you_  for a minute in p- _private?”_

“Uh—”

Dr. Betas tugged Sans away down the hall and into another room before Sans could protest further, closing the door behind them. Gaster stared at the hallway for a moment. He could hear them murmuring, but they were too far away to make out.

Gaster closed his eyes and let his Determination take over. He brought a hand up over the left side of his face and tried to focus, searching for the correct timeline. It was eavesdropping, but he wanted to hear this.

Slowly their voices came to him.

“…just can’t believe you.” That was Dr. Betas, or at least a very close version of him. “You don’t have to do this for him again. I-I mean, what are you, his, h-his trained hunting dog or something?”

“That’s not fair.”

“Th-The last time almost  _killed_  you, Sans. Not b-because of any danger you were in, but because of what it  _did_  to you. Maybe he d-didn’t see it, but I did.”

“You’re exaggerating. And he’s not  _that_  fargone, Betas. He’s not asking me this time. And he was always good about not…talking about what I’d done, after. M…Mostly.”

Gaster sat down in his chair. His head hurt, but he kept listening.

“You r-really can’t tell when you’re being manipulated?”

“Of course I can. Please. How long have I worked with him now? I can  _tell_  when he’s… The point is, he’s not doing that this time. You saw his face when I offered. He looked at me like I’d burst into flames.”

“Sans, c-come on, let’s just…we can convince him to wait if we t-try harder.”

“Yeah, I don’t think so. Even if we did, I’m…I don’t know what he’d do if he had to wait more.”

“Please, you can’t do this to yourself. Wh-Why do you go so far for him? You _know_  he wouldn’t do the same for you.”

“I…I like to believe he would.”

Gaster buried his face in his hands. He tried to convince himself it was from guilt. The truth was simply that his head was hurting. Knowing that made everything hurt worse.

“What if it’s a kid again?”

There was a sad, nervous chuckle from Sans. “Then…then it’s a kid again. Then I guess…I save the King a lot of heartache, huh? Heh, or…heart-‘break.’ Heh…yeah. That’s not funny. That’s…that’s the really horrible thing, isn’t it? Kid or not, they just. Someone’s gonna kill them eventually. That’s just h-how it’s gonna go. We need the souls. Four is better than seven, right? It’s…better, right?”

“Sans…please, I-I’m your friend. You don’t have to do this. I-I’d rather Gaster just go do it than you. Look at you, you’re shaking.”

“Betas, you’ve…have you ever killed anyone?”

“What? N-No, of course not!”

“Well, here’s the thing. It…it does something to you. Not really magical or anything. It’s just this…after you’ve done it, you really realize that you’re—you’re capable of anything. That you can do anything at all under the right circumstances. Even if you’ve gone your whole life thinking you’d never do something like that.”

“That’s what I’m saying! You’ll j-just—”

“No, let me finish. The thing is, that feeling—I’m—I’m scared of what will happen to Gaster if he’s the one to go out and kill this human. He’s…he’s slipping, Betas. It’s getting worse every day. And I’m trying so hard to hold onto him, but he’s getting away from me. If I let him go out and kill a human, I think…I think that’ll be it. I think he’ll become something we don’t even recognize. And if he actually absorbed the soul, with the way he is now? Can you…can you imagine that? Can you imagine what he’d be like?”

Dr. Betas had no answer for that. Gaster peered out between his fingers, watching the hall and the door behind which his scientists were hiding.

Gaster felt something he hadn’t felt in a long time. Shame.

Sans sighed.

“Listen, let’s not worry about it, okay? Worry is exhausting. For now I’m just gonna be another pair of eyesockets looking out for a human, yeah? There’s no guarantee I’ll even find them. And if I do, I’ll just call the guards. That’s all. It’ll be okay.”

“Just…just t-take care of yourself, Sans. Alright?”

“Always do.”

Gaster sat up straight in his chair as the two of them left the room and walked back down the hallway toward him. He schooled his expression and let the glow in his eyes fade. His Determination settled. His focus narrowed.

**“Has Dr. Betas managed to talk you out of it?”**

“Nope,” said Sans, walking past Gaster toward the elevator.

**“You are…not going to change your mind, are you.”**

“Nope.”

**“Sans…this is…perhaps I was too…”**

Sans hit the elevator button and stepped in, turning back to meet Gaster’s gaze. He looked like he was waiting for Gaster to say something. Gaster wished he knew what to say, but there was only one thought coursing through his mind.

_Go fetch me that soul._

In the end, Gaster said nothing. The elevator door slid closed.

 

***

 

Sans started at the Waterfall-Hotland border, where the human had last been seen. There was a water cooler there that tended to collect a lot of monsters, tourists or otherwise. It was always a good place for gossip.

He eased his way into the conversations taking place and eventually managed to start asking around about a human who had come through here. Half the monsters there had no idea what Sans was talking about, and since one of the points of all this was trying to avoid a panic, Sans backed off. Most of what he heard were obvious lies, up to and including the theory that the human had already killed Asgore and was secretly ruling the entire Underground. Eventually, Sans managed to separate the lies from the more interesting rumors, and from there he tracked the rumors back to a somewhat reliable source.

“Oh sure, I saw the human,” said a very cheery rock monster. “They came by here a few days ago. But don’t spread it around—I know people don’t really trust humans, but this one was very nice! Very polite.”

“Yeah?” Sans was good at acting casual and only mildly interested. It was one of his stronger points. “Neat. Nah, I won’t spread it around. Man…what’s a human even look like? I’ve never seen one.”

“Ha! Of course you haven’t! Who has? They’re rare, you know. Anyway, he—I think it was a he?—was covered in weird flesh and cloth and had this silly little tuft of fur on his head. Like a shaved dog! So funny.”

Sans chuckled at that. That did pretty much describe a human, at least in his experience.

“Where’d they go after that?”

“Oh, he said he was looking for a job! Can you imagine? My friend told him to check around at that inn up near the Core.”

Sans knew exactly what inn the rock monster was talking about. He chatted for a few more minutes before making some excuses and wandering off.

It seemed the human had gotten deeper into Hotland than anyone had really expected. Gaster’s theory must have been true. A human would never make it this far on their own—someone must have been helping them. Maybe lots of someones. Monsters tended to be forgiving sorts. Even if humans had trapped them all here long ago, and even if the most recent human had killed at least one of their kind, the average monster could be persuaded if the human acted nice enough. Some monsters just didn’t understand the concept of deception. The human had probably charmed their way into the heart of Hotland. It was also likely that half the monsters between Snowdin and here had no idea that they had met a human at all.

Sans’s ribs started to hurt in a familiar, nervous way as he approached the inn. Tracking down this human might be less difficult than he thought it would be. He had no idea what he was going to do if he saw the human just randomly working behind the counter at the inn. How was he supposed to explain this to anyone? Especially if this turned into an open fight. Sneaking up on a human was one thing, but trying to face one in a direct fight was rather…terrifying. He might have blue magic these days, but he was just an average monster, and he was as weak as he had always been.

And of course, there was the very real chance that this human would turn out to be just like the last one. Just a scared, helpless kid lost in the Underground who didn’t want to fight at all.

Eventually he reached the inn. The human wasn’t there; Sans couldn’t help a quiet sigh of relief. The innkeeper, however, claimed to have seen them.

“Sure, at least, I think it was a human,” the small dragon said, distracted with other guests. “I’ve never seen one before, now, have I?  _I_  don’t know what humans look like. But I had an odd sort of monkey-monster here the other night. They stayed one night, paid and left.”

“Don’t suppose you know where they went?”

The dragon grumbled, clearly busy and overworked.

“Sorry, man. I’m on Royal Guard business.” Sans flashed his laboratory badge. It wasn’t good for much more than opening doors, but at least it looked pretty official. “You know how it is. Gotta keep the bosses from yelling at you too much, yeah?”

“Of course, of course, the safety of the Underground and all that,” the dragon said, waving a clawed hand. “They said something about looking for a job, so I sent them toward that spider bakery. Heh, maybe she’ll just eat it and save us all the trouble, right?”

Sans grimaced, said his thanks and left.

Muffet’s bakery was in the opposite direction. Sans had only been by there once before; Muffet was okay, if extremely creepy. Monsters tended to avoid the entire spider-controlled area unless they had a strong craving for their, admittedly, excellent pastries. Muffet’s prices were exorbitant, and there were rumors that she had once eaten a monster who had stolen from the tip jar. Sans was inclined to believe it.

Never mind that the cobwebs always stuck to your feet when you went in.

“Humans? Ahuhuhu! Don’t be silly, dearie! I think I’d notice if a human came by!”

“Yeah?” Sans hid his annoyance behind a smile as he forked over 100G for a tiny croissant. A few spiders carried it out of the display case, wrapped it in tissue paper and handed it to him.

“Do you know what humans look like? I’ve never seen one.”

“Certainly! They’re like spiders with fewer legs, aren’t they?”

“Hey, random question…don’t suppose you have any job openings here? Got a friend who’s looking for a job—I figure a café’s a good place for him to start.”

“Oh, I’m sorry, but this is a family owned and operated business,” Muffet said with a fangy smile, propping her chin on two of her hands. “You know, though, you’re the second person in two days to ask about a job! I had to send some poor monster off just yesterday.” She made a face. “He didn’t even buy anything.  _Very rude.”_

“Yeah. Kids these days, right? No manners, heh. Did you let the poor guy down easy?”

“Oh, I  _did_  try. Sent him along to try his luck at that…” Muffet bared all of her fangs. “Delightful little… _restaurant…_ down the road. The one that is taking quite a lot of business away from me! Do you know, I heard that that place isn’t up to fire code! Ahuhuhuhu…anything could happen.”

“Heh. Uh. Yeah. Anyway. Thanks for the croissant.”

“See you around, dearie~ Do come back soon!  _Do.”_

And so it went. Sans went from place to place, always two steps behind the human. The day wore on. Sans hadn’t realized that detective work would be so _exhausting._ He was starting to feel like he was on a wild Temmie chase, and he was no closer to actually finding the human. All he had was a very vague description. It sounded like the human wasn’t a child this time—thank goodness for that—but other than that, all Sans had was that the human was probably male, and might have been carrying a frying pan.

Eventually night fell, which only ever resulted in a slightly deeper darkness shrouding the cavern ceilings above Hotland. Sans decided that he would try a few more leads, then head home and call Gaster to give him the bad news. The technically  _good_  news. Gaster wouldn’t be happy, and Sans might have to try again tomorrow, but at least today…well, at least today he wouldn’t have to fight and potentially kill some poor human.

Maybe it was cowardly to hope that the Royal Guard would just deal with this, but Sans was fine with being a coward. He didn’t  _want_  to kill a human. Not again. Despite what Gaster always tried to tell him, Sans couldn’t stop regretting what he had done to the human with the blue soul. They’d been a child. A scared, lost child who hadn’t deserved to die afraid and alone.

But all of the alternatives this time around were a lot bleaker than the prospect of having to dirty his hands again. Gaster couldn’t be allowed near a living human—not in the state he was in these days. Sans knew with crushing certainty that Gaster’s Determination wouldn’t be able to pass up the opportunity to gobble up a tasty human soul.

Then Gaster would truly be lost.

Sans made one of his last stops at a small, greasy restaurant on one of Hotland’s main thoroughfares. He’d been here before; some dank little hole in the wall that sold really crappy burgers and, horror of horrors,  _stale_ ketchup. Nowhere near as good as Grillby’s. But he might as well grab a bite to eat with whatever information he could gather.

“You want the special tonight?” said the growly unicorn at the counter. “It’s fancy.”

“I don’t think I could  _stomach_  your specials, heh.”

The unicorn glared and jabbed a hoof-like finger at Sans.

“Don’t turn up your nose-hole at it before you even know what it is, skeleton.”

“Fine, wow, okay. Humor me, then. But if it’s as bad as your burgers, you’re gonna owe me double the info in exchange, capiche?”

The unicorn snorted. “Judge all you want, but I’ve got a new chef who’s working some real magic around here.  _Finally._ The special is quiche.”

The unicorn slammed a plate with some kind of pie slice on it in front of Sans. Sans raised a brow ridge. It certainly  _looked_  better than what this place usually put out.

“What’s…quiche?”

“Like an omelet in pie crust.” The unicorn leaned toward Sans in what he probably hoped was a threatening manner. Really, he just looked like an Aaron who was flexing too hard.  _“Try it.”_

Sans took a very, very tentative bite.

It was actually…good. Savory. The crust was buttery and flaky, and the filling had a perfect eggy texture.

“Whoa. Okay. This is—uh. Not terrible.”

The unicorn leaned back and whinnied in satisfaction.

“See! See! I told you! Let’s see you judge me after this one! Oho, that flame monster in Snowdin is going  _down.”_

Sans prodded his slice of quiche with a fork, frowning vaguely.

“You said you have a new chef?”

“Well, it’s not like I suddenly improved in my cooking! Ha! Ha.”

“Could I meet this miracle worker, maybe?”

“Sure, sure. Hey, kid!” The unicorn cupped his hoof-fingers around his snout. “Hey kid, get out here! A very judgy so-and-so wants to give you their compliments!”

Sans heard a clatter from the kitchens in the back. A knot of tension formed behind his sternum. The quiche was delicious, but all at once Sans had lost his appetite. The kitchen door swung open and someone poked their head out.

Sans squinted. It… _looked_  like a human, but…

Ah.

Yes. It was a human. A boy, somewhere in his teens, wearing a stained apron and holding a frying pan. He had on a sort of headband with two pompoms that looked a bit like some kind of insect’s antennae. Sans hid a panicked little smile. Of course. Of course something that ridiculous had fooled all these monsters.

The human’s gaze fell on Sans and he blinked in mild surprise. Sans gave a tiny wave.

“Oh, um.”

“Hi. Uh. Uh, so…you’re the new…chef here, huh?”

“Y-Yes, sir. I just started yesterday. Um…I hope I’m doing well so far?”

Sans resisted the urge to drag his hands down his face.

“Yeah, you’re…heh. You’re doing great things, kid.” Sans tapped the quiche with his fork so he didn’t have to keep looking at the human. “This is actually good. First good thing I’ve had here.”

“See! Compliments! Finally!” the unicorn crowed in triumph.

“Th-Thank you, sir,” the human said, clasping his hands in front of him, his face turning a bit pinkish.

“Don’t mention it. Really…don’t even mention it. Actually, kid, you mind if I—talk to you for a sec?” Sans glanced at the manager. “It okay if I borrow him? Just for a bit.”

“Hmmmm! He’s very busy! Lots of orders to fill!”

Sans looked around the restaurant. It was empty. The unicorn fumed.

“Okay! Okay fine! But no funny business, skeleman!”

The unicorn shot him the stink eye before finally stalking away into the kitchen. Sans waited until he was sure the unicorn was out of earshot.

“Um, so, I’m really glad that you liked my cooking, sir…”

“Kid, let’s just—let’s just cut to the chase,” Sans said, pushing the quiche aside and rubbing the spot between his eyesockets. “You’re—you’re a human, right?”

The teen jerked back as if Sans had just brandished a knife at him. He wrung his hands, his eyes flicking left and right.

“What? Heh, haha, that’s funny, I’m d-definitely not a human. See?” He pointed at the fake antennae. “Humans…don’t have these…right?”

“Yeah, that…might have fooled most of the folk down here, but not me. I’ve actually seen a human before. Nice, uh…headband, though.”

The human looked miserable.

“I’m…I didn’t want to lie to anyone, I just…thought if I looked less human people would stop trying to fight me… My boss doesn’t know. Please don’t tell him?”

Sans wanted to laugh. The human’s boss finding out was the least of the kid’s concerns now. Sans didn’t know what to do. He could summon a few bullets right now, end it quickly. Now that he’d seen the kid’s face, though, he didn’t think he could do it. Nor was he sure how he would explain it to the unicorn, or any other monsters who happened by.

He had to call Gaster. Or the Royal Guard. But what if the teen tried to run?

“Nah, I won’t tell him. Uh…listen, we…you…”

Sans could usually talk his way through any situation, but right now he was at a loss.

“Do we have to fight now?” the boy asked, fidgeting with his frying pan. “I don’t even know how to fight. Everyone kept trying to fight me until I put this dumb headband on. I don’t want to fight.”

“I’m…n-no, kid, I’m not gonna fight you.”

“Oh…oh, thank goodness.” The boy’s face broke into a smile. “I was really worried there! You kind of—you kind of looked really intimidating for a second.”

“Look, kid, can we—can we talk in private?”

“Oh, sure! I’ll tell my boss I’m taking a break and we can go out back. Is that okay? Heh, there’s, there’s not much to do here tonight, is there? You know, this is my first job ever. Um, okay. Here, we can just go right through the kitchen.”

Sans hesitated. What was he doing? What was he  _supposed_  to do? He needed to be calling the guards  _now._ He already had a hand around his phone in his pocket. The guards would show up in a matter of minutes and Sans could pass the responsibility off onto them. He couldn’t do this. He couldn’t just have a  _chat_ with this kid and then murder him in cold blood.

He waited until the human’s back was turned, then slid off the stool to follow him. The kid said something to the unicorn and the unicorn bellowed a response. Sans slipped his phone out of his pocket and texted Gaster as quickly as he could.

_found human. restaurant called the greasy hoof. send guards._

Sans pocketed his phone and followed the human through the kitchen. The unicorn was nowhere to be seen, which was fine by Sans. The fewer monsters around right now, the better.

The kitchen’s back door led out into an alleyway that mostly contained dumpsters. The alley was dark. Sans kept his eyesockets on the human, alert for any sudden movements. The human put up a good front, but this one could turn out to be just as nasty as Gaster always said they were. A dark alley was the perfect place to dispose of a nosy monster, after all.

The human stopped and turned back to Sans. He was still holding that frying pan. The kid dusted off his hands on his apron, then reached up and pulled off the pompom headband.

“Is this okay? It’s kinda gross cause of the trash and stuff.”

“It’s fine…”

“Um, so, you know, you’re one of the only monsters I’ve met who actually knew what I was. So other humans have come down here, right? There’s, um, there’s always been rumors about people disappearing at Mt. Ebott. It’s supposed to be haunted. No one ever returns.”

Sans had heard some of those rumors. They had originated a very long time ago, back when the first human had fallen down.

“Yeah. You’re the fourth. Er…technically the fifth.”

“Wow…have you met all of them?”

“No. Just one.”

“So what happened to them?” the human looked worried. Sans looked away before realizing what a bad idea that was. He couldn’t let this human get the drop on him.

“They, uh. They’re gone. They died.”

The boy drooped a little. Sans felt his phone vibrate in his pocket. He could only hope that was Gaster with an answer. He didn’t dare look at his phone now. Not while the kid was watching.

“I had a feeling…” said the human, turning his frying pan over in his hands. “There were plaques on the wall in that marshy place. It had this…whole story about the war and the barrier. They teach us that stuff in school, but they teach it…kinda different. Um…so…I saw the part about…the seven souls.”

The ache behind Sans’s sternum came back and spread through his bones. He felt his eyelights go out.

“I…I had a feeling this would happen. She warned me about this…I should have listened…”

“’She?’”

The kid kept going as if Sans hadn’t spoken.

“But you said you didn’t want to fight, right?” The teen clutched his frying pan to his chest.

“I don’t, kid. I really don’t. But it’s—I—” Sans had no idea what to say. “It’s…this whole thing is just so…complicated.”

“I mean…I understand.” He was shaking now. He looked like he was fighting back tears. “It’s not fair. All of you being trapped down here. Everyone knows the war was a huge mistake.”

“…Yeah?” That was news to Sans. “Humans think it was a mistake?”

“A lot of people do, I think. But there’s nothing we can do to fix it from up there. So no one really talks about it.” The boy paused. “What if…what if you just borrowed my soul?”

“It doesn’t really work like that.”

He and Gaster had wondered about that back in the day—if maybe a human’s soul could be removed while their body was kept in stasis, so they could avoid actually having to kill humans. Nothing had ever come of it. It simply wasn’t possible.

“I don’t want to die.” The boy sniffed, but seemed to be trying to force himself not to cry. “I really like it here. I like it so much better than home. Even when monsters tried to fight me, I just talked to them and they ended up being so nice. All of you are so  _nice._ E-Even you.”

Sans gritted his teeth. Nice. Here he was, debating whether to murder the kid, and the kid called him  _nice._

“At first I wanted to go back, but I think I could r-really live down here, you know? I got a job and all. No one complains about me cooking. My dad always hated it. My mom, though…” He hugged his frying pan. “She’d be proud. I was thinking of all kinds of new recipes…there’s a lot of cool ingredients down here. I just…I just want to cook food for people. That’s all. I don’t want to die. Can’t…can’t you just let me go? Can’t you wait for another human? I-I haven’t done anything wrong. I know it’s n-not fair that all of you are trapped here, but i-it’s not fair that I have to die, either. I-I’m only fourteen…”

“Kid, stop.” Sans covered his face with his hands. “Just stop.”

He couldn’t do this. He couldn’t kill this kid. He couldn’t keep pretending that this human was faking all of his kindness. This human was  _genuine._ He had no LV, no killing intent whatsoever. There was no evil in his soul. Sans just couldn’t do this. He couldn’t do this again. He had tried so hard to rationalize killing the child in Waterfall, more than a year ago now. The fate of all of monsterkind, weighed against a mere handful of humans…it should have been obvious what the right course of action was. It was simple numbers.

Just one more soul. Just one more and they could Reset, and then no more dead humans, no more trapped monsters, no more barrier at all. The soul was right in front of him, and…

Sans couldn’t do it.

He was so terrible at this. All that speechifying back at the labs. Pathetic.

“Kid, I…”

The human was staring at him with fear. Beneath the fear was the tiniest glimmer of hope. Hope that Sans would spare him.

“Listen, I…I already called the guards.”

“Oh…” The boy wiped at his eyes with his free hand.

Sans pulled out his phone. This was insane. He had to be insane. He checked the phone; there was a text from Gaster, in Wingdings as usual.

_Understood. Guards are on their way._

The text was five minutes old.

“They’re on their way. If I know the Royal Guard, and I do, I’d give it about five more minutes.”

He looked up at the human again, deciding.

This was going to be the stupidest thing he had ever done.

“Maybe—maybe if you—”

His phone buzzed in his hand. What timing. It was another text from Gaster.

_Sans. Do you think that I cannot see you right now?_

Sans stared at his phone.

“M-Mister skeleton? Are…are you letting me go?”

The phone buzzed. Another text. Then another. Then a third. Then a fourth.

  _Do._

_Not._

_Betray._

_Me._

Cold fear such as Sans had never known before flooded through his bones.

“Mister?”

Sans’s mind had gone blank. He couldn’t think. He couldn’t breathe. His bones felt tight. It felt like everything was coming apart around him.

The human took a step back. Sans extended a hand. There was a  _ding_  and the human’s soul turned blue, the glow appearing at the boy’s chest.

“What…wh-what did you do?”

“I’m sorry, kid.”

Gaster always knew exactly what to say. Sans almost started laughing. This was just like last time. He had let himself be manipulated again. What an idiot he was.

Trained dog, Dr. Betas had said. Sans curled his fingers.

“I’m so sorry. I don’t want to do this. But I think…heh. I think he might actually kill me if I let you go.”

The boy struggled backward, but the blue magic slowed him down too much. He started crying in earnest. Sans could hear a commotion in the distance. The guards were coming.

“I’m…so sorry.”

“It’s okay. It’s okay.” The boy gave a watery smile. “I forgive you.”

That hurt worse than anything else had tonight.

“You shouldn’t.”

“Mom said to always be kind. So—I forgive you.”

Sans said nothing else. He closed his eyes and waited for the guards.


	10. Into the Dark

**Summary for the Chapter:**

> The team tests out the time machine and Gaster reveals his plans.

**Notes for the Chapter:**

> Originally posted at my [Tumblr](http://talkingsoup.tumblr.com/thescientistfanfic).

The green soul glowed warm in its container, thrumming gently against Gaster’s fingertips. There was Save power here, and plenty of it. He stared into the green light for a few, longing moments before carrying it over to the machine. His hands trembled as he slotted the green soul into a compartment next to the others.

Four souls. At last.

 **“Let’s begin the test,”**  Gaster said, turning to Dr. Betas.  **“We will start at 2% power and slowly increase it from there. We must be careful not to overheat the machine.”**

Dr. Betas fidgeted with his hands. He kept glancing nervously in the direction of the elevator.

“Sh-Shouldn’t we wait? Shouldn’t S-Sans be here for this?”

 **“Let him mope for awhile.”**  Gaster stepped over to a console near the machine. Tubes and wires snaked back and forth between the two. Gaster flicked a few switches and a few lights on the machine came on.

“I’m worried about him…it’s been over a day and h-he hasn’t even called.”

**“He will be fine.”**

Sans had vanished immediately after turning the human to the guards. He hadn’t contacted Gaster or anyone at the lab, and he hadn’t gone home. Papyrus had actually called earlier looking for him; Gaster had simply lied and told him that Sans was hung up at work. Sans needed some time, and Gaster could understand that. And if Sans was still gone after another day, Gaster could simply go collect him. He knew exactly where Sans was—holed up in a little corner of Waterfall. Gaster had been keeping an eye on him, in a sense, to make sure that Sans didn’t try anything else foolish. It was a simple enough thing these days to peer through time and space and locate the skeleton. Gaster’s left eye had been so helpful lately. There were even times that he could actually control it.

Gaster turned a dial on the console and the machine hummed to life. The souls began to glow brighter. Gaster smiled. This was the beginning.

 **“We will increase the power output by increments over the next six hours,”**  he said.  **“Another five hours and the machine should have absorbed enough of the souls’ energy. Then the testing will begin in earnest.”**

Dr. Betas was already typing away on his computer, monitoring the machine’s progress.

“Everything looks good so far…”

 **“Of course. We have done everything right. Every eventuality has been accounted for.”** Gaster folded his hands behind his back, watching the machine and its glowing souls.  **“We have come too far to fail now.”**

***

Sans was back into work the following day. Gaster smiled to himself when he heard the elevator descending, then grinned openly when Sans walked out.

 **“There you are, Sans.”** Gaster gestured expansively at the fully charged, fully operational time machine.  **“We were forced to begin without you. You are just in time for the first test. We shall certainly need your expertise.”**

“Sans!” Dr. Betas rushed across the room and stopped just short of actually tackling the skeleton. “I was s-s-so worried! Dr. Gaster said you were o-okay, but… And Papyrus e-even called us, h-he had no idea where you w-were, he was worried and we h-had to tell him you were h-here…”

Dr. Betas wrapped Sans in a tight hug. Gaster watched them from the other end of the lab. This was a waste of time, but he could allow them their emotional little moment. They wouldn’t have many of those left, after all.

“Whoa, hey, Betas, it’s okay.” Sans patted Dr. Betas’s back gently. “I’m fine. Just needed to get away for a bit, that’s all. I went home last night, so don’t worry about Paps either. Sorry for not calling.”

“The human didn’t hurt y-you?”

“Nah.” Sans tugged himself out of the hug. “I’m fine. Not a scratch on me, see?”

Dr. Betas glanced back in Gaster’s direction, then whispered something else to Sans. Gaster narrowed his eyes, but it was too late. He had missed it, whatever it was. Sans responded with a small shake of his head.

“So.” Sans patted Dr. Betas on the shoulder and headed toward the machine, giving Gaster a wide berth. Now that he was closer, Gaster could see that the skeleton clearly hadn’t slept at all. He was more disheveled than usual.

“First test, yeah?” Sans stopped in front of the machine and put his hands on his hipbones. “Should be interesting. Charging it up wasn’t a problem?”

**“None at all.”**

“Some unusual hull vibrations b-but nothing really unexpected,” Dr. Betas said, coming to join Sans.

**“Your designs to cause resonance between the souls and the machine’s systems proved most effective, Sans.”**

Sans shrugged. “Just a matter of matching up sine waves. Not hard.”

Gaster came to stand behind them, giving them both space. It wasn’t like he had forgotten the last thing he had said to Sans before Sans had disappeared into Waterfall. In retrospect, perhaps he had overreacted. But it was either that, or allow Sans to let the human go. That simply was not an option.

Gaster wondered if Sans finally hated him. That might present a problem. The bigger problem, however, was whether or not Gaster could still trust Sans at all.

Sans looked past the machine. The four human souls were in their facets on the wall. Cyan, orange, blue and now green.

“Turned out to be green, huh?”

“Y-Yeah,” Dr. Betas said, rubbing the back of his neck. “The, uh, guys in m-magic tech were very excited. Green magic is the most powerful human soul-based magic they’ve s-seen.”

“Yeah…?”

 **“Its effects are twofold.”** Gaster took a careful step closer to them. Sans finally turned and looked Gaster in the eye. Gaster paused.

Sans didn’t just look exhausted. He looked…

Resigned.

Sans stared up at Gaster. “…Go on. I’m listening.”

Gaster cleared his throat.

 **“It affects monster bullets as well as souls,”** he said.  **“Green bullets take on a healing property. Incredible, really, that a human’s soul could contain healing magic.”**

Sans gave a small, tired smile.

“Heh. Incredible.”

**“When used on a soul, it pins the soul in place, similar to blue magic. The affected host then cannot move at all, and thus cannot attack, forcing the host to stand firm and defend. Also rather odd, for a human.”**

Sans broke eye contact with Gaster, turning away to walk over to his desk. He practically collapsed into his chair. Sighing, he brought up the current data on the time machine and started looking it over.

“Kid did say he didn’t want to fight at all.”

Gaster didn’t believe that for a second.

**“In any case, we have a test to run. Shall we begin?”**

“By all means, Doc. Things look good over here.”

“A-Alright then.” Dr. Betas grinned as he walked over to the machine. He might be a nervous wreck lately, but even he was excited at the prospect of finally testing what he probably considered to be his life’s work. He spun the wheel on the machine’s hatch and pulled it open. There was a hiss as the hermetic seal was released. Dr. Betas looked up at the other two and held up a ballpoint pen.

“This is one of my good pens. S-So let’s hope this works, right?”

“It’s a giant step for pens everywhere,” Sans said, giving him a thumbs-up. “You think its pen-pals will be proud?”

Dr. Betas snorted and set the pen down on the floor inside the machine. He seemed to be savoring the moment. Gaster wished he would hurry up. His fingers were already hovering over his computer’s keyboard.

Dr. Betas shut the hatch and turned the wheel to seal it. He stepped back to his work station.

“Kinda wondering if we should have a blast shield,” Sans said. “Bit late for it but—”

 **“My magic will protect us. Don’t worry, Sans.”** Gaster looked pointedly in Sans’s direction.  **“None of us will come to harm.”**

Sans didn’t look at him. Gaster stifled a sigh and turned back to his computer.

“R-Ready.”

“Ready.”

**“In three, two, one.”**

Gaster typed in a command. The machine powered up with a high-pitched whine. The whine quickly became louder until it was a shriek that seemed to vibrate in Gaster’s mind. He winced and the images in his left eye shattered and multiplied until he was seeing thousands of time machines at once. He felt his eye come alight with magic. Pain seemed to echo through his eye and up the crack in his skull.

Gaster squeezed his left eye shut, trying to focus. Brilliant light flashed from inside the machine, accompanied by crashing sounds like muffled lightning. The machine shook. For several long, tense seconds, it seemed as if it was simply going to shake itself to pieces.

Then all at once it stopped. The light inside it vanished and the machine powered down. Gaster typed a few commands on his computer as the machine’s program began generating progress reports. The ache in his head slowly faded; Gaster opened his eye again.

“Well, dang. That was louder than I thought it would be.”

“That m-might have been too much power.”

**“Only one way to find out. Open it up.”**

Dr. Betas headed to the machine again, Gaster and Sans close behind. He gingerly laid a claw on the hatch wheel.

“It’s a little bit warm to the touch. I-I hope the heat shielding is holding up.”

**“Open it.”**

It had worked. Gaster was sure of it.

“M-Moment of truth!”

Dr. Betas turned the wheel and pulled open the hatch. Gaster’s smile vanished. There on the machine’s floor was a puddle of melted plastic, ink and metal. It was sparking with white monster magic.

“Oh dear…”

Sans stepped back, cursing under his breath.

Gaster felt his hands curl into fists of their own accord.

“W-Well, the first test was bound to be a failure, right?” Dr. Betas darted away to go find something to clean the mess. “W-We can adjust the power output and—”

 **“Are you quite certain this wasn’t a hardware failure?”**  Gaster said, tracking Dr. Betas’s movements.

“I-I mean it’s possible, but my designs—”

**“This should have worked.”**

He felt a tug at the sleeve of his lab coat and looked down to see Sans.

“Ease up, Doc. This was just the first test. Like he said, it was bound to…Doc, your eye.”

Gaster blinked and reached up to cover his left eye. The glow hadn’t faded. Gaster gritted his teeth, trying to bring himself back under control.

Dr. Betas returned with a beaker and a bit of cardboard. The remains of the pen had cooled, giving the puddle a viscous quality. Dr. Betas managed to scrape the mess into the beaker, then sealed it.

“You gonna be alright, Doc?”

He could feel Sans watching him, waiting to see what he would do. Gaster wanted to snap at him. It seemed that Sans intended to keep on babysitting him, despite what Sans had tried to do. The hypocrite. But Gaster couldn’t yell at him. He needed Sans on his side.

**“I am fine, Sans.”**

He forced himself to calm down. Slowly the glow in his eye faded. He opened it again, taking a deep breath.

“I’ll run s-some tests on this,” Dr. Betas said, giving the beaker a small shake. “Maybe—”

 **“No. We will make some adjustments, lower the power output and try again,”**  Gaster said, going back to his desk.  **“Check the reports generated at your stations and make the proper fixes.”**

“Doc, slow down. We—”

**“We are running out of time.”**

“Heh, isn’t that kind of the idea?”

 **“Really?”** Gaster spun on him.  **“You are _really_  making jokes right now?”**

Sans stared at him for a moment, then sighed and gave a noncommittal shrug.

“It’s what I do.”

**“Well I have no patience for it right now.”**

“You have no patience for  _anything_  right now.”

**“Do not test me, Sans.”**

“No, right.” Sans chuckled a little. “We’re testing the  _machine,_ not you.”

Sans sat down at his desk and said nothing more. He went to work, clicking through some of the status reports on his computer. Gaster glared daggers at the back of Sans’s skull.

“Um, so…” Dr. Betas stood midway between the two of them, clutching the beaker and looking awkward. “Should I—?”

**“Label the beaker and set it aside. We will analyze it later. Bring another pen.”**

***

Gaster had stopped taking thorough notes on his work. He hadn’t written a full log in over a year. These days, his notes consisted of short, summarized entries in his notebook, and he often forgot to do even that. Everything he needed to know was already stored, in multiple forms, in his own mind. There seemed little point in writing anything down. Soon, this timeline would be gone. It wasn’t like he planned on bringing all of his notebooks back to the past with him. What would he possibly need them for?

Still, force of habit. He kept a short record of the tests on the time machine. Encoded in his own unique language, naturally.

 

Entry 2

           Machine cracked the lab floor halfway through test. Resulted in subject (ballpoint pen) melting into magic-infused ooze again.

           Failure again. We don’t have time for this. In a very literal sense, we do not have time.

           Lost temper. Broke chair with bullets.

           Chair? Wasn’t it a desk?

           No. Definitely chair. Perhaps in another timeline it was a desk (?)

           Sans was able to bring me down. Still quite angry with him for interference with regard to green soul. However, I am consistently forced to admit: he always knows what to say to pull me back.

           I will need him when I go through the dark.

 

Entry 4

           Took further precautions after last test’s short-circuit. Tests continue to leave subjects infused with magic. Odd. Subject (glass beaker) turned into sand. Progress??

           Feel calmer today.

           Asgore contacted me. First time talking in months. Was good to hear his voice.

 

Entry 5

           Why are the subjects infused with magic regardless of their original properties/the effects of Reset on them? This might present a problem when we move on to monster subjects. It doesn’t make sense. Possible side-effect of attempting to convert Reset into magic?

           Test subject (lead cube) melted.

           Checked Core again. A matter of days.

           Can feel my Save. Like an itch in the back of my mind. Almost wish I had never created it.

           Dr. Betas was acting suspicious today. They are always suspicious lately.

           What does he know? What are they hiding?

 

Entry 7

           It has been three days.

           Subject (iron ball bearing) melted. Infused with magic again.

           Can feel the Core. If it triggers before we are finished

           No time no time no

           Stay Determined.

 

Entry 9            

           Sans is the only one in this forsaken timeline who matters.

           And even then

           EVEN THEN.

           He still infuriates me.

           Subject (?????) melted, infused with magic.

           Lost temper. Broke something?

           Sans was there to keep me sane. Sane? Is that what this is?

           Dr. Betas had some kind of silly screaming fit. Would not stop YELLING. Left. Has not returned. Pathetic pathetic always knew he wasn’t worth my time.

           Sans is all I need.

 

Entry 10

           Breakthrough!

           Subject (notebook, college ruled) remained intact. Was still infused with magic. It seems that the subject becoming infused with magic is not preventable. Perhaps it will prove to be beneficial.

           Measurable tachyon particle activity within subject as well. Extremely promising.

           I am surprised that Dr. Betas was not present for this test. Nor did he join us later that night at Grillby’s. The machine would not have been possible if not for his skills as an engineer. Perhaps he is out sick?

           Or perhaps he is aware that soon he will no longer exist.

 

Entry ?

           Which timeline is this?

           Subject (mouse) did not survive the test.

           What a mess. Had Sans clean it up.

           HE SEEMS TROUBLED BY SOMETHING.

 

Entry 13

           Mistook the intern Alphys for Dr. Betas. Almost brought her down to the basement lab. Fortunately Sans corrected me. That would have been interesting to try and explain.

           Subject (mouse) did not survive. Less of a mess this time. Progress?

           The Core weighs heavily on my mind.

 

Entry 14

           Subject (mouse) survived the test but vanished soon after test’s completion. Disappeared into thin air. Presumably a result of tachyon particle overload. Subject has either traveled into another timeline or has been scattered among multiple timelines. No way of knowing.

           Everything seems to be going g o ing  GOING GOING GOING

           SHUT UP SANS.

           My notes seem to be a bit jumbled

           Or perhaps

 

Entry 16

           Subject (mouse) survived unharmed. Continued to show no ill effects after three

           No

           CAN’T TELL WHERE

           There was a small earthquake just now

           It is starting.

 

Entry 17

           DARK DARKER YET DARKER

           THE DARKNESS KEEPS GROWING

           THE SHADOWS CUTTING DEEPER

           PHOTON READINGS NEGATIVE

           THIS NEXT EXPERIMENT

           SEEMS

           VERY

           VERY

           INTERESTING

           …

           WHAT DO YOU TWO THINK?

 

***

 

Sans had not had a full night’s rest in two weeks now. Between the constant work with the time machine and his sheer inability to sleep, he had only managed to get a few hours at night and a nap here and there during the day. Over the last few days, Gaster had become completely intolerant of Sans’s naps, banging on his desk to wake him up whenever he drifted off.

The nights were always worse. At the end of the day he had to go home and face Papyrus, and every day Papyrus would ask how his day was, and every day Sans would lie. It was fine. Everything was fine. And yet Sans would lay in bed and stare at the ceiling and think about dead human children, or the violently shattered bodies of test mice, or of Gaster’s bullets throwing Dr. Betas against a wall, breaking his arm in three places.

The worst part wasn’t that Dr. Betas had had a nervous breakdown immediately thereafter and had quit the science division altogether. It wasn’t even Dr. Betas’s scream when it happened, or the sound of his bones breaking—the worst sound Sans had ever heard in his life. No, the worst part was that Gaster hadn’t even meant to do it.

The doctor had lost his temper, as he had done countless times over the past year. Sometimes he even summoned his bullets by accident, but even then, Gaster had always managed to keep a modicum of control. He had never fired his bullets at anyone—until now. And Dr. Betas had paid the price. He was lucky to be alive. He was lucky it hadn’t been a Gaster Blaster.

Sans had talked Gaster back down, like he always did, once he had been sure that Dr. Betas was physically stable. Gaster had seemed so confused afterwards. As if he hadn’t realized what he had done. Five days later he still didn’t seem to really understand. He kept addressing Dr. Betas as if he was still there.

Dr. Betas, at least, was doing much better. Sans had visited him at home. Sans hadn’t seen the poor guy that relaxed in years.

“At least one good thing c-came out of all this, right?” Dr. Betas had said. “Green magic makes it so much easier to use healing spells. M-My arm should be usable again in a few days.”

“Hey, I’m…sorry again. I couldn’t stop him in time.”

Dr. Betas shook his head, looking regretful.

“I’m just glad you stopped him before he could…keep g-going. And at least it wasn’t o-one of his Blasters, right?” Dr. Betas sighed heavily, sinking lower in his armchair. “It never should have g-gotten to this point, Sans.”

“Yeah…”

Dr. Betas looked at him with worried urgency. “You need to get out of there, Sans. While you s-still can.”

Sans folded his hands in his lap, lacing his fingerbones together.

“I can’t. He’s still my friend. I think I can still help him.”

“You’re trying so hard to save him, but w-who’s gonna save you?”

Sans had no answer for that.

“Just pr-promise me that you’ll be careful?”

“Heh. Always am. Anyway…we should probably stop talking about it. He might be watching me.”

The lack of sleep didn’t matter. The lies to his brother didn’t matter. Even the time machine didn’t matter. At this point, Sans no longer cared whether it worked or not. All that mattered was pulling Gaster back from the pit before him. Sans had to at least try.

The days since Dr. Betas quit had been rough. Gaster slipped away faster and easier. There were long stretches of hours during which he muttered to himself or communicated entirely in summoned bullets. Even when he talked normally his voice was becoming harder for Sans to understand. Sans had never had trouble with Gaster’s speech before now.

On the final day, the day of the eighteenth test, Sans arrived to work early. He didn’t know what had prompted him to do so. Afterward he would describe it as dumb luck.

Gaster was waiting for him in front of the elevator when the doors slid open on the basement level. The doctor smiled at Sans, his hands folded behind his back.

**“Welcome, Sans. Are you ready?”**

Sans smiled back despite his exhaustion. “For test eighteen? Lucky eighteen, right? You never actually explained what this test would entail. We moving onto monster subjects?”

Sans didn’t see anyone else in the lab, though, and it wasn’t like Gaster would be taking volunteers for a top secret, experimental project that could end with the monster melting or exploding. He looked past Gaster to where the time machine stood. It had been unhooked from the wall. He had a sinking feeling that Gaster was going to ask Sans to climb in there.

Or maybe he wouldn’t even ask.

 **“In a sense, but not quite. This next experiment will be…quite a lot more involved.”** Gaster leaned down so he could put both hands on Sans’s shoulders.  **“Sans, I do not tell you this enough, but I have been eternally grateful for your help and loyalty. Your assistance has been indispensible these past few years.”**

Sans blinked up at him. Whatever he had been expecting…this was not it.

“Oh.” He rubbed the back of his skull, heat climbing to his cheekbones. Gaster almost never sincerely complimented people. “I, uh. Thanks. I mean. It’s no problem. Er. I mean…”

Sans smiled a little, his first genuine smile in several days.

“It’s really been a pleasure working with you.”

**“This experiment is going to change everything, Sans. It will be like nothing we have attempted thus far. We are going to Reset everything.”**

“Already?”

**“Yes. Everything is ready.”**

The ground trembled. Sans started, looking around the lab. Fixtures shook on the walls and glass equipment rattled together. Another tremor? The Underground got earthquakes sometimes, but this was the second tremor in a week.

Gaster looked up as if he was seeing something else.

**“It is time. I need to know that you are with me, Sans.”**

“I…yeah. Till the end.”

 **“And beyond that, I hope!”** Gaster chuckled a little, though Sans didn’t understand why. It wasn’t often that he missed a joke.

Gaster let go of Sans and stepped away, turning back toward the machine.

**“Now. Let’s get the machine onto a maglev cart. We have a bit of a walk.”**

“What? We’re taking it out of the lab?” Sans followed Gaster. “Is that safe? Where are we going?”

**“To the Core.”**

 

***

Sans pushed the machine while Gaster walked on ahead. He had been to the Core a thousand times before—you couldn’t get from Hotland to New Home without passing through the Core—but he had never been down this route before. There had been the usual amount of monster traffic on the road to the Core, and quite a few people had stopped to stare at them or ask them what was going on. They made quite a spectacle; the tall Royal Scientist walking ahead and not acknowledging anyone, and a short lab assistant pushing a ten-foot-tall machine floating a few inches off the ground on a maglev cart.

Sans waved off inquisitive Pyropes and Vulkins and Royal Guards by saying they were conducting a complex science experiment. Most people stopped listening when you mentioned science.

Once they got to the Core the traffic thinned out, which was strange. The Core was usually thick with monsters—if not travelers, then staff. But there seemed to be hardly any staff working today, which never happened. Sans had never seen the Core so empty and quiet. He couldn’t help but find it ominous.

“How deep are we going?” he finally asked when they had descended a third elevator.

 **“As deep as we can.”** Gaster glanced back over his shoulder at Sans, smiling faintly.  **“Tiring out already, Sans?”**

“You know me.” Sans mopped his forehead. “I’m always tired.”

**“Do not worry. We are almost there.”**

Sans looked around. This part of the Core was almost unrecognizable. The lighting was different from the main area, and there were none of the usual puzzles. The staff had completely vanished, leaving the Core seemingly devoid of life. Their footsteps echoed in the metal hallways.

“I didn’t even know this area existed,” Sans said, puffing a little as they stopped in front of yet another elevator. The maglev cart took away about 98% of the machine’s weight, but it was still heavy and difficult to navigate.

 **“There are many areas of the Core that no one still living knows about,”** Gaster said, pressing the elevator call button.  **“I built many such places with my own two hands. So long ago, now. The purpose of the Core was always twofold. It provides power for the entire Underground, certainly. But the Core was meant to be so much more than that.”**

Sans could hear the elevator clanking its way up from somewhere below. He leaned against the levitating machine and pulled out his phone to send a text to Dr. Betas. Dr. Betas wanted nothing more to do with Gaster or the experiments, but he had also wanted to be kept up to date. Just in case Gaster did something rash.

“So what’s the second thing, then?”

**“Hm?”**

“The second thing. You said the purpose of the Core was twofold.”

 **“Oh. Right.”**  The elevator pinged and the door creaked open. Gaster stepped aboard and Sans followed, pushing the time machine.

**“The second purpose is what lies in the very bottom of—”**

The floor began to shake, cutting Gaster off. Sans flailed and braced himself against the wall of the elevator. The elevator swayed to the side; Sans could hear it straining on its cables. The tremor continued for another few seconds before fading. The entire Core seemed to groan.

“Holy—two in one day?”

Gaster laughed and hit the only button in the elevator. The door slid closed.

**“It’s getting feisty! Very soon, now.”**

“Wait…are you saying that whatever’s down there is  _causing_  the earthquakes?” Sans stared at the floor, wishing he could see through to the bottom. The elevator began to descend.

**“It is waking up. Sometimes I can almost believe that it is alive.”**

Sans watched Gaster’s face. The doctor was smiling broadly. The elevator creaked as it descended. Sans looked at the floor again, starting to frown. There was a pressure growing behind his sternum, as if his nonexistent heart had started to pound.

“What’s really down there, Doc? What were you going to say before?”

**“The Core is a power station, but it is also an engine. A magic engine, designed to concentrate ambient monster magic. Every time a monster has used their magic or summoned their bullets in the last thousand years, the energy left behind seeps into the air and ground. The Core is a geothermal power station. It does not just draw energy from heat and steam—it also siphons off that magic left behind and condenses it, gathering it in the room below.”**

Gaster turned to Sans and grinned.

**“I wish you could have been a part of this process, Sans, but I had to keep it a secret. You understand. But, oh, you would have found it all so very fascinating. Did you know that when condensed, monster magic changes color?”**

Sans was growing nervous now. He could feel a cold sweat starting to prick along his bones.

“I…well, isn’t that how colored magic works?”

**“Sort of. But when you concentrate magic past that point, it turns into something else. Not black, not really. It turns _dark._  A lack of not just color, but also light. I took some readings yesterday and found the photon levels to be completely nonexistent. Well—I needn’t say more. You will see.”**

The elevator came to a stop and pinged. The door scraped open.

“That’s all pretty fascinating…” It  _was_  fascinating. Sans could write an entire thesis on these revelations. “I don’t get what this has to do with time travel and Reset, though…”

**“Follow me.”**

Gaster stepped out of the elevator. The room beyond him was virtually empty. There was nothing but a catwalk suspended above what Sans could only assume was the very bottom of the Core. It was warmer here than anywhere else; they had to be very close to the magma here. As Gaster stepped onto the catwalk, lights sprang on at strategic points along the railings. Overhead lights flickered to life as well. The ceiling was at least twenty feet above the catwalk.

Sans pushed the time machine out of the elevator. The catwalk was wide enough to allow the floating machine between the railings, still leaving a gap on either side. Once he was out, the elevator closed again and Sans was finally able to see the floor, several dozen feet below.

Only there was no floor.

There was nothing below the catwalk. No floor, no ground, no magma. The walls of the cavernous room descended into darkness. Everything beyond that point was pitch black, so dark it was like a hole in the world.

Sans froze. He had never been afraid of heights, but the darkness below him made him feel like he was standing above an  _impossible_  abyss. He let go of the machine and gripped a railing with both hands.

“What…Gaster, what…where are we…?”

Gaster laughed.

**“Sans, you should see your face!”**

Sans gripped the railings tighter, shaking a little. He couldn’t stop staring into it. The darkness below seemed to draw in light and devour it.

 **“Should I have warned you not to look down?”** Gaster was still chuckling. **“Don’t worry, Sans. You are not going to fall.”**

“How is this possible?” Sans couldn’t bring himself to let go of the railing. He closed his eyes so he didn’t have to see it anymore. “That’s  _magic?”_

**“In a sense. Really, Sans, it’s not that bad. Let go. We need the machine to be positioned in the center.”**

“I—sorry. Just. Give me a second.” Sans gritted his teeth. “It feels… _wrong.”_

He could  _feel_  it. The darkness was churning, even if it was impossible to see. It was like a heaviness on his mind.

**“Nonsense. Come along. We don’t have time for this.”**

Sans forced himself to open his eyesockets. He inhaled sharply when he saw the darkness again and tore his gaze away from it. This was wrong. Something about this was so wrong. What was this… _thing_  doing here? What were they supposed to do with it?

Something gripped his hand. Sans flinched, startled, but it was only Gaster prying one of his hands away from the railing.

 **“I know it is unsettling,”**  Gaster said, voice stern.  **“But you said you would follow me. Until the end. Follow me now.”**

Gaster gave him a tug. Sans took a breath and let go of the railing. Gaster kept hold of his hand and led him away down the catwalk. Sans hauled the time machine after him.

“You need to explain this to me, Doc,” Sans said, rather breathless. “I don’t understand. Why are we here? Why is  _that_  here? The Core has been…what, producing this thing the whole time?”

**“Exactly correct. For over a thousand years now.”**

Sans glanced downward again, then immediately looked away. He tried to focus on Gaster. His hand was solid in Sans’s own. Even so, it was difficult not to feel like he was falling. The closer they got to the center of the room, the more Sans could feel the hole below them. It felt so  _empty._

“But what for?”

**“For this day. Here. Turn off the maglev and lower the machine onto the catwalk.”**

At the center of the catwalk was a wider area with a control panel that presumably could retract the catwalk back toward the opposite walls.

Sans did as he was told, tugging his hand out of Gaster’s. The machine sank onto the metal grating of the catwalk. Sans shuddered as he felt the catwalk tremble.

**“Don’t look so nervous, Sans! You look as nervous as Dr. Betas. Where is the old lizard, anyway?”**

“He, uh. He quit, remember?”

**“Ah. Right.”**

“Doc.” Sans went back to holding onto the railing, just for security. “Tell me. What is all this? What are we doing?”

**“The machine will produce Reset energy and will allow the occupants to travel back in time safely. That is what the tests up until now have all been for.”**

“I get that, but—”

**“The problem was always finding a pathway back to the beginning. Reset is not enough. It can only take one back so far. I told you I had the issue of the pathway covered, did I not?”**

Sans remembered that conversation. He looked down. Gaster couldn’t  _possibly_ be implying what he thought he was implying.

Gaster spread his hands over the darkness.

 **“This is our pathway.”** He grinned.  **“Magic condensed to the point of darkness. Dense enough to punch a hole through spacetime, allowing someone to travel anywhere—or, excuse me, any _when—_ that they wish. There is a similar concept in astrophysics, up on the Surface. When certain stars die, they collapse on themselves, leaving a gravity well from which nothing can escape. These ‘black holes’ are theorized to be tunnels to different points in spacetime. Ha! Of course, any attempt to enter a true black hole would result in the subject being reduced to as single stream of particles. The gravity is too strong—survival is impossible. This concentrated magic is much less deadly, though a lot less stable. Originally, my plan was simply to enter the darkness when it was large enough. But I soon discovered that doing so without proper shielding would result in almost certain death. A vessel would be required. A vessel in which the occupant could be safe from the tidal forces of magic and spacetime alike. Why do you think I was so pleased with your time machine designs? And why do you think I was so adamant about making sure it worked without damage to the occupant, inanimate or otherwise?”**

Sans stared down into the dark, trying to process what Gaster was saying.

“You’re…” He closed his eyesockets and rubbed his forehead. “You’re saying that you’re going to…fly the machine  _into_  that.”

Gaster beamed at him.  **“Exactly! The dark magic provides the pathway. The machine takes care of the energy required, and of course, the destination. But not just me, Sans. _We.”_**

Sans’s eyelights shrank to pinpricks.

“What?”

Gaster took hold of both of Sans’s hands.

 **“I want you to come with me, Sans.”** Sans could feel him trembling with excitement.  **“Back to the beginning. To the war. You will be indispensible in the past, Sans. You have grown so strong.”**

“I—I’m not  _that_  strong…” He wasn’t strong at all. Powerful, maybe, but not strong. “Gaster, this is…how do we even know this will work? All we’ve proven so far is that the machine works. And—it might not  _keep_ working. We haven’t even tested it with monster subjects yet. How do we know it will hold up under the stress of— _whatever_  that is down there?”

**“I have run numerous tests in secret, Sans. I have full confidence that this will work. It is a shame that we could not test the machine with monster subjects, but we have run out of time.”**

“Yeah…y-you keep saying that.” Sans pulled his hands out of Gasters’. He took a step back, trying not to let his fear show on his face.  _“Why_  do you keep saying that? I always…thought it meant because eventually the Determination would get the better of you, but…it’s more than that, isn’t it?”

Gaster studied him with a mild expression. He was silent for a few long moments, staring through Sans. Another tremor rumbled through the Core, making the catwalk shiver. Sans held onto the railing for dear life. When the earthquake passed, Sans turned away and slipped his phone out of his pocket, carefully so as not to alert Gaster. Gaster was distracted enough that he didn’t notice Sans sending a text to Dr. Betas.

_something is wrong. gather everyone you can. bottom of the core. might need to stop him._

**“As perceptive as always, Sans,”**  Gaster said eventually.  **“The Determination was part of it. I can only assume that sometime soon I will lose control entirely. Though if I am quick, it might not even matter. It might even prove to be strategic, depending on what all this Determination will finally do to me.”**

“Gaster,  _why_  are we out of time? Just  _tell me.”_

 **“Ah…well, you may not like this part, Sans.”** Gaster looked almost sheepish. Sans’s grip on the railing tightened until his knuckles hurt.

_“Why are we out of time?”_

**“The darkness has reached maximum capacity,”** Gaster said, turning away and staring down into the abyss.  **“The earthquakes will soon increase in number and intensity. When we activate the pathway and the machine begins to resonate with it, the Core will destabilize. The resulting explosion of magical and geothermal energy will destroy the Underground. I am no geologist, but I can only assume that most of the caves will collapse. Magma will flood the tunnels and rise.”**

Gaster started laughing quietly.

**“Ah, but the truly humorous thing—humans thought that trapping us under a mountain, with access to the heart of the planet, to the magma—we are only about a mile from the Earth’s mantle, did you know? Humans thought this was a good plan! And here we are, about to turn a mountain that should have remained a simple mountain for the remainder of its lifespan into a volcano! I wouldn’t be surprised if the mountain simply blew itself in half from all the pressure. Every human for _miles_  will die.”**

Gaster shrugged, still chuckling.

**“Isn’t that hilarious, Sans? They trap us down here, and a few thousand years later, we send the _literal underground_  right back up to meet them! Of course, every monster here will die as a result, but that seems a fair enough trade-off. Ah, I remember in the war, so many of them thought we were demons or some such nonsense. ‘Go back to hell!’ They screamed that as they slaughtered my family. ‘Go back to hell!’ And now here we are, bringing hell to them! Isn’t it funny, Sans? Why aren’t you laughing?”**

Sans lurched backward, one hand clutching his chest, the other covering his mouth. He felt sick. He stared up at Gaster in horror.

Every monster. Every human for miles. Thousands upon thousands of people.

Gaster rolled his eyes.

**“Don’t give me that look, Sans. Haven’t you figured it out by now? None of that is going to matter. You and I will be safely in the past. Once we Reset, this entire timeline, everything here, everything that has happened or is going to happen—it will all be destroyed. Erased. That is how this works. To create a new timeline, the old one must be deleted. Like moving a few files on your computer to the recycle bin.”**

There was no earthquake, but Sans was shaking all the same. It felt as though the world was coming apart around him. His breaths came fast and shallow. Terror constricted around his bones.

“No…no, no, this…” Sans clutched the sides of his head. “This can’t be  _real._ You can’t…you can’t be that fargone…”

Gaster frowned and took a step toward him. Sans scrambled backward and came up against the door of the machine.

**“Listen to me, Sans. It is not like anyone will really die. You have no need to worry about that. When we Reset, it will simply be that none of this ever existed. No barrier, no Underground.”**

“When…when the hell did you decide that this was an acceptable option?”

Gaster sighed in exasperation and folded his arms.

**“I decided all of this a very long time ago. I have long since made my peace.”**

“You’re talking about  _genocide!”_

**“Did you not hear me? I just said that no one will die. In fact, I assume that most of the monsters we know today will continue to exist in the new timeline’s future—just in vastly different ways. Don’t be melodramatic, Sans.”**

“You have no way of knowing that! And we will be stuck in the past…in the middle of the  _war._  Assuming the two of us could even do  _anything_  to stop the humans—”

Gaster grinned, and there was no mirth there. A simple baring of teeth, vicious.

 **“I believe that I will be able to handle them quite readily,”** he said in a low, dangerous voice.  **“And you will be able to back me up. You have both types of blue magic. They won’t even know what has hit them.”**

This couldn’t be real. This had to be some kind of nightmare.

“So…s-so you’re not just asking me to erase the timeline where everyone I have ever cared about lives…you’re not just asking me to help you destroy every living thing in range of the Core’s explosion…you’re also asking me to come back to the past, not…not for the pursuit of science, not to have an assistant, but because…because you want me to help you slaughter humans. Because I’ve gotten  _good_ at it. Am I…am I getting all that right?”

Gaster rubbed at his head as if Sans was giving him a headache. Vaguely, Sans hoped that he was. This was crazy. How could Gaster be serious about this? How could he have planned it all this way since the very  _beginning?_

 **“That is hardly the reason, Sans.”** Gaster stared at Sans, his expression somewhere between frustrated and…confused.  _Confused._  He was  _confused_  about why Sans was reacting so strongly.

“Then  _what?”_

**“You are the only thing in this timeline worth sparing, Sans.”**

Sans blinked at him for a moment in shock, then ground his teeth together and glared down into the abyss below.

“No,” he bit off. “No, you don’t get to  _say that to me._  Everything you’ve done, everything you’ve asked of me…I can’t believe it. I can’t believe how stupid I’ve been. The only thing worth  _sparing?_  But this was your plan from the beginning. Since before you even  _met_  me.”

**“No. That is not true. I did not understand the true nature of Reset until I made myself Determined. And when I created my Save, that was when I fully realized. The goal has always been to return to the past and stop all of this from ever happening. But it was not until recently I realized that it would result in this timeline’s destruction.”**

Sans thrust a finger at him.

“Oh! Oh, but destroying the entire Underground by turning the mountain into a _freaking volcano,_  that,  _that_  was always on the table!”

**“I am growing tired of your melodrama, Sans. Yes, that was the very real eventuality when I first built this place. But again, it would not have mattered. My Resetting the timeline would have prevented that from ever happening. Have I not said this already? Do try to pay attention.”**

Sans had never been so angry and so frightened in his entire life. He couldn’t even remember the last time he had even raised his voice.

“You keep saying it won’t matter! But what if all of this doesn’t  _work,_  Gaster? What if we do everything right and it just  _doesn’t work?_ Did you just trap us in a timeline in which the Underground is about to be  _destroyed?”_  Sans stopped short, narrowing his eyes. “No…you’re not that stupid. You’re not stupid at all. There…must still be a way to stop this. Am I wrong?”

Gaster didn’t answer, but the look on his face was that of a dangerous animal still deciding whether to bite.

“I knew it.” A relieved smile came to Sans’s face. “I knew it. There’s a way to stop this. We can shut all of this down.”

Gaster tilted his head.

**“And why would I, exactly?”**

“Because I’m asking you to.” Sans spread his hands. “I told you I would be there to pull you back, yeah? This is me pulling you back. I’m  _begging_  you, Gaster. We can’t do this. We can’t doom the entire Underground and any humans up there. We can’t just erase everything. You  _have_  to see how insane this is. Doc, please.”

Gaster didn’t answer, but Sans saw his eyes flick to the side. He was having doubts. He had to be.

Sans kept going.

“Please.  _Think about this._  We either end up in the past, and everyone here dies, and we’re stuck with a war on our hands. I don’t…I don’t want to kill anyone else, Gaster. Human, monster. I’m _sick_  of it. It’s either that or…or it doesn’t work, and we’re stuck here. We’re stuck here and everything we’ve worked toward doesn’t matter because everyone’s dead. Either way, everyone’s  _dead.”_  Sans covered his face with his hands. In his mind he could  _see it_ —miles and miles of gray dust and broken human bodies. The whole world destroyed. “Everything and everyone we love gone. Dr. Betas and Alphys, King Asgore, the whole science team. My brother…oh god, my  _brother_ …”

**“Oh _forget_  your idiot brother!”**

Gaster stormed forward, crossing the distance between them in two strides. Sans tried to back away, forgetting he was against the machine. Gaster towered over him.

 **“It’s always your _stupid_  brother,” **Gaster snarled, his left eye coming alight with magic.  **“I am _so utterly sick_  of hearing about  _Papyrus._  I could never understand it. Is his stupidity meant to be charming? Is his complete lack of  _any_  useful, redeeming qualities meant to be _endearing?_ How can you care about someone  _so completely pointless?”_**

“Sh—”

 **“I knew that skeleton would prove to be an obstacle. _Pathetic.”_  **Gaster pressed one hand to his head, fingers clawing against his scalp. He looked wild.  **“I should have had him _killed_.”**

Something terrible and hollow opened inside Sans.

“Ga—”

 **“You would have been useless for awhile, but you would have gotten _over it.”_** Gaster spread his arms wide, eye sparking.  **“All he has _ever_  done is stifle you, Sans. All he has ever been is a  _burden.”_**

“Shut up.  _Shut up.”_ Sans tried to shove Gaster backward. Gaster didn’t even stumble. “You don’t know a  _goddamn_  thing.”

**“It is the truth. You have simply never been willing to admit it.”**

“You don’t get to talk about him like this! You don’t get to act like you somehow have  _my best interests_ at heart when you’re about to  _kill_  everyone!”

Sans tried to shove Gaster again. This time, Gaster caught Sans by his wrists and pushed him against the machine, pinning him in place. Sans let out a yelp of pain as the hatch wheel hit him in the spine.

 **“Pathetic,”** Gaster said, voice soft.  **“Just like everyone else. Too attached to something so transient. From a certain point of view, I suppose you are right, Sans. Everyone is about to die. Papyrus included. When he’s gone, perhaps you will finally listen to me.”**

“I’ll stop you.” He gasped as Gaster pushed him harder, making the wheel dig in between two of his vertebrae. Any harder and his back would break.  _“Ngh,_  please, Gaster, just  _stop this._  You can stop this. W-We’ll think of something else, some other way t-to break the barrier…”

Gaster dug his thumbs in between the small bones in Sans’s wrists. Sans hissed in pain and struggled, squeezing his eyesockets shut.

**“I can, but I won’t. I have come too far. Worked too hard. I cannot stop now.”**

“Then I really  _will_ stop you. Th-That’s supposed to be my job, right?  _Hhhh,_ heh. Funny. Don’t remember that in the original job description.”

**“How do you plan on stopping me, then? You are powerful, Sans, but not that powerful.”**

“Dr. Betas is coming,” Sans growled, opening one eye. “Hopefully he’s not alone.”

 **“Is he, now?”** Gaster dug his thumbs in deeper, until Sans cried out.  **“So in the end, you really did betray me.”**

“I’m trying t-to  _save you,_ jackass.  _Think about this._  Think about what you’re _doing.”_

**“I have had a thousand years to think about it.”**

“Hey, Doc.” Sans’s head lolled against his chest. He peered up at Gaster and grinned a little. “Did…d-did you hear the one about the doctor who  _fffff—_ who fell into a well?”

Gaster blinked at him.

**“I beg your pardon?”**

“Yeah. He should have focused on the sick and left the  _well alone.”_

_Ding._

Gaster’s soul turned blue and Sans shoved as hard as he could with his magic. Gaster didn’t even have time to react. He flew backward, tumbling head over heels, coming to a stop twenty feet down the catwalk.

Sans didn’t waste a second. He dropped to a crouch and whirled, switching the maglev cart back on. The time machine rose into the air. Sans darted around to the other side, grabbed hold of the edge of the cart and started pulling it back toward the elevator as fast as he could. If he could just get the time machine out of here, buy some time for Dr. Betas to arrive…

**“SANS.”**

“Oh god oh god oh god…”

Sans pulled faster, half-running, but the time machine was too heavy. The elevator was too far. He could hear Gaster’s footsteps rushing toward him. Gaster wouldn’t use his bullets, not so long as the time machine was between him and Sans.

Sans let out a shout of surprise as he saw a stream of hand-shaped bullets go shooting by past him. Apparently he had been wrong. The bullets crashed into the wall next to the elevator, leaving dents in the metal.

**“GET BACK HERE, YOU CALCIFIED, PATHETIC EXCUSE FOR A MONSTER!”**

Sans didn’t dare look back over his shoulder. The elevator was so close now.

_Ding._

Sans stopped short, as if he had been frozen in place. He went rigid, completely unable to move. With an effort, he shifted his eyelights downward.

There was a green glow at his chest.

Gaster rounded the time machine and hit the button on the cart to drop the machine again. The cart was wrenched from Sans’s still outstretched hand. He tried to move—nothing happened.

Without even hesitating, Gaster grabbed Sans by the collar of his lab coat. He lifted Sans off the catwalk as if he weighed nothing, then tossed him away down the catwalk. Sans hit the metal hard. Sans tried to scramble to his feet, but halfway there Gaster simply took hold of his soul again. The feeling was a thousand times worse than blue magic. It was like his soul was in a vice grip, as if Gaster had physically wrapped his hand around Sans’s soul.

Sans looked up at Gaster. Gaster came to stand over him, one hand outstretched. His face was virtually unrecognizable.

 **“A fair enough attempt,”** Gaster said, almost sounding impressed.  **“But at the end of the day, you are a failure.”**

“G…Gaster…let me go.”

There was a sound. A sound that Sans knew too well.

A Gaster Blaster appeared. It opened its mouth and seemed to stare at Sans through glowing eyesockets, almost as though it was alive. Everything in Sans screamed at him to  _run,_  but he couldn’t move. He couldn’t  _move._  He stared into the Gaster Blaster’s jaws, at the point of light already gathering in its throat.

**“A failure, but still useful. A shame it had to come to this, but I need you, Sans.”**

“No—please—”

**“We are going to Reset now. You will come with me into the past. You will help me continue my work. You will do as I say, everything I ask of you, even if I have to _permanently_  take control of your soul to do it.”**

“Gaster,  _please.”_  Sans tried to move, but it was as if his bones had simply stopped working. He stared up at Gaster, meeting his eyes. “We’re friends. I’m your _friend.”_

**“You’re my _science project.”_**

Sans felt something break inside him. Not a bone. Something else.

**“Yes. Nothing but a science project.”**

“That’s…”

**“A simple question I asked myself. Can I take this pathetic creature and make him _useful?”_**

“Please…”

**“What did you _think_  I saw when I looked at you, Sans?”**

“That’s  _not true.”_

**“I saw what everyone else sees. A small, lazy, useless excuse for a monster. Worthless. You were so happy when I gave you that power. So happy that you could finally be of use to someone. So happy that you could finally protect your equally worthless brother. What a sad little thing you must have been up until that day. The day _I_  gave you power. The day  _I_ fixed you. And this is how you repay me.”**

“Stop. This…this isn’t you. I know we—we’re—”

**“There were so many experiments I wanted to try on you. I can’t for the life of me understand why I held back. Perhaps you can assist me with some of my theories after we have Reset, Sans. Once we have finished with the humans, I can turn my attentions to you fully. Will you still think of me as your friend when I have broken you apart and turned you into something _truly_  wonderful? My little scientist turned science project. But don’t worry. I will keep you alive until…”**

Gaster trailed off and frowned.

**“What is that?”**

He leaned closer.

**“Are you _crying,_ Sans?”**

Sans tried to reach up to mop the tears away, but nothing happened. He never cried. Not in front of people. Not in front of anyone but Papyrus.

It really was pathetic. The world was coming to an end, everyone he had ever known was about to die, and yet  _this_ was the thing that broke him. He was Sans. He laughed things off, because the alternative was  _this._ Crying solved nothing. It made everything hurt worse, and it showed the people around you just how  _weak_ you really were.

He could handle this from total strangers. He had handled it all his life. But hearing it from Gaster made all of this, everything that was happening, become all at once real. Everything was going to end. Gaster would pull Sans into the past and make Sans do exactly what he wanted. And Sans…was just Sans. A small, weak monster who couldn’t stop anyone. Not even his friend.

Maybe Gaster was right. Maybe they had never really been friends.

 **“Why would you…?”** Gaster shook his head as if trying to clear the cobwebs from his mind. His lip curled. Another tremor rolled through the Core.  **“It doesn’t matter. We are out of time.”**

Light continued to gather in the Gaster Blaster’s jaws.

**“If you won’t come willingly, I will merely have to knock you out and force you. I suppose it will be easier for everyone if you are unconscious.”**

“You’re going to kill me,” Sans said quietly, staring down the Blaster’s throat.

 **“Don’t be silly. I have more control than that.”** Gaster smiled again, but there was a bit less vitriol in it this time.  **“It will certainly hurt like hell, though.”**

Sans saw one last chance. One last glimmer of hope.

“If you shoot that thing at me, you’ll kill me.”

 **“Do you mean to tell me you won’t just dodge?”** Gaster chuckled.  **“Oh, right! You can’t dodge if you can’t move. Seems your one skill is useless, Sans. Do be a good monster and hold still.”**

The Gaster Blaster’s lower jaw split in half and it opened its mouth wide. The sound of gathering energy rose in pitch.

“No, you…you don’t understand.” Sans stared into the light. “I’ll die. E-Even if you hold back. I’ll be dust the—the second you fire. C-Can’t turn dust into a science project, can you?”

**“Stop talking. Hold still.”**

“Gaster, stop. Stop!”

**“Just close your eyes. This is going to hurt.”**

_“Gaster!”_  Sans screamed his name.  _“I’ll die! I’ll die in one hit! I only have one HP!”_

The sound of gathering magic plateaued. The Gaster Blaster stopped with its jaw unhinged, mouth full of light.

**“Excuse me?”**

“I-I only have one HP. P-Please. Please s-stop.”

Gaster frowned. The Gaster Blaster hovered at his side, ready to burst.

 **“Absurd,”**  Gaster said after a moment.  **“I made you strong.”**

Sans made a snarling sound, furious, desperate, in pain.

“You made me  _powerful,”_  he spat. “You  _never_  made me strong. I’m as  _weak_  as I always was. You  _bastard._  You never even  _noticed._  You saw I could use magic and _patted yourself on the back._ Job done, right? You complete  _bastard._ You s-stand there, ready to kill everyone, ready to erase the whole world. Ready to kill  _me._ And you didn’t even know it. You say I’m the only thing worth sparing in this timeline, then you say I’m  _worthless,_  just a  _science project_  to you. You try to hurt me, and now you  _hesitate_  when I tell you I’ll actually die.”

Sans’s glare bore into Gaster’s eyes.

“You hypocrite! Make up your  _goddamn mind!”_

Gaster pressed a hand to his head.  **“I…”**

“Come on, Doc! What are you waiting for? Are you going to  _kill me_  or not? What am I gonna be to you, huh? Your friend? Your  _science project?_ Or a pile of  _dust?”_

The Gaster Blaster vanished.

**“No, no…this wasn’t supposed to…”**

Gaster turned away, clutching his head and doubling over as if in pain. Sans looked down at his own chest. Still green.

“You were fine with destroying the whole world a second ago,” Sans’s said venomously. “What’s one more monster? Don’t kid yourself, Doc. You don’t really need me back there. All those human armies and wizards. You can take them all down yourself. Or you’ll find some other sadsack to torture and manipulate. Heck, if you wait long enough, you’ll even find another Sans. Then we can do this all over again. You gonna kill him too?”

**“Shut _up._ Stop  _talking.”_**

Another tremor shook the room, making the catwalk sway.

“Come on, Doc. You’re out of time, remember? I’m not going with you. So you might as well just kill me.”

**“I…I don’t… _nnrrrgg…”_**

“You don’t what?” Sans let his voice soften. Almost. Almost there. “You don’t what, Gaster?”

Gaster gave a sad, strangled little noise.

All at once, the green glow at Sans’s chest vanished. He slumped back against the catwalk and curled into a shivering ball. The tears almost started again, just from sheer relief.

He peeked up at Gaster. The doctor was staring at his hands as if he had never seen them before.

 **“I…”** He stopped. Looked around. He seemed to take in the machine, the catwalk, the darkness below. Finally his gaze settled on Sans.

 **“Sans…? What…what happened? I…”** He clutched at his face.  **“What have I done? What did I _say?_ God…oh my God. I could have…Sans, I almost killed you…”**

“Yeah…” Sans rested his skull on the warm metal. “Got you to stop, though.”

Gaster took a step toward him. Sans scrambled backward and Gaster halted, watching Sans with a piteous look. He held up both hands, as if to show he meant no further harm.

 **“I am…so sorry, Sans.”** Slowly, Gaster crouched down. Even slower, he offered Sans a hand.  **“I am so sorry.”**

Sans looked between Gaster’s face and the offered hand. He started to reach forward, then hesitated, eyelights flicking to Gaster’s face again. Gaster made no further move toward him. His expression was pained. Heartbroken.

Sans reached out and took Gaster’s hand. Gaster pulled him onto his feet, then forward wrapping his arms around Sans. Sans tensed, certain it was another trick, but…

Gaster was hugging him. He was shaking.

**“I am so sorry. For everything.”**

“Hey…” Sans’s arms were too short to wrap around Gaster entirely, but he hugged back all the same. His fingers curled into Gaster’s lab coat.

“Hey, there he is.” Sans leaned his head on Gaster’s shoulder, face breaking into a sad smile. “Welcome back, Doc. It’s been awhile. I missed you.”

 

***

 

Gaster held onto Sans as if he was the last real thing in the world. Perhaps he was.

It had happened at last. Gaster had lost it. He had lost  _everything._ He felt clear again for the first time in years. It wouldn’t last long. This was it.

The end.

The Core shook again, more violently this time. The catwalk groaned and swayed. Sans flinched in Gaster’s arms. Gaster held him tighter. There was a whirring sound as the elevator began to ascend. That would be Dr. Betas. He and whoever else had come to stop Gaster would be here in less than a minute.

**“No…”**

It was too late for them now. Too late to warn them.

**“What have I done?”**

“Hey.” Sans pulled back, just enough so that he could look Gaster in the eye. Gaster’s heart broke all over again upon seeing Sans’s face. The skeleton looked drained, exhausted, frightened. Broken. “There’s still time. We can stop it. Right? Just tell me what to do.”

**“I think…that I am finished telling you what to do, Sans.”**

“Doc—”

Gaster let go of him and stood. His left eye flickered nonstop, so he closed it. No distractions. He assessed. Analyzed. There were only a few seconds left to make a decision, and his options were sorely limited. The elevator had started to descend. No. No distractions. Dr. Betas and everyone aboard that elevator were already dead.

He looked at Sans. He looked at the time machine. He looked down into the dark.

 **“There is something I can do,”**  he said.  **“But—”**

The world shook. The catwalk pitched upward, then down again as the wave cascaded through the Core. Gaster and Sans were thrown off their feet. There was a loud bang from the opposite end of the room as one of the struts connecting the catwalk to the wall came loose. The entire catwalk began to tilt.

“Whatever you’re going to do, do it fast!” Sans said, clinging to the railing as the catwalk leaned further and further to the left.

**“I believe I can utilize my Save.”**

“What, you’re…you’re gonna Reset back to your Save?”

**“Not quite.”**

Another earthquake. A second strut gave out with a scream of metal. The darkness swirled below them. Gaster wasn’t looking at it. He was staring at his hands. His fingers had begun to melt together. Determination had finally caught up with him.

**“Time is up.”**

Gaster extended a dripping hand toward the time machine, tipped over and wedged now between the railings. Hand-shaped bullets took hold of the hatch wheel and spun it, pulling the hatch open.

“What are you doing?” Sans cried, wrapping both arms around the railing as the catwalk began to sink and buckle. “You can’t—the timeline—!”

 **“I’m not.”**  Gaster could feel the rest of his body starting to melt. He could no longer feel his legs.  **“I can seal the darkness. Stop the destabilization. But you, Sans—you need to survive.”**

“No, no, don’t you dare—don’t you—”

The catwalk groaned and buckled and Sans lost his grip. He fell down toward the darkness. Gaster let go of the railing and extended his hand. Sans’s soul turned blue and he stopped in midair, only a few feet above the dark.

“Gaster!”

 **“Remember your promise, Sans,”** Gaster said, bracing whatever was left of his feet against the railing.  **“Now is the time for you to quit.”**

“No, wait—”

Gaster yanked on Sans’s soul and tossed him upward into the time machine. He slammed the hatch closed behind him. Another burst of magic and the machine activated.

The elevator door opened. Dr. Betas and several others—scientists, guards—began to pour out.

  **“Stay back!”** Gaster cried, but it was too late. Two of them stumbled forward and fell, disappearing into the dark.

“Oh G-God!” Dr. Betas screamed, spreading his arms to keep anyone else from leaving the elevator. He looked up at Gaster. “Doctor, you need to stop this! Dr. Gaster!”

 **“I am,”** Gaster said, gritting his teeth and tearing his eyes away.  **“I am so sorry, Dr. Betas. I will not be able to save you.”**

Metal tore and began to shred. Gigantic cracks ran up the walls from the darkness below, splitting the elevator shaft to pieces. Sections of wall came loose and tumbled into the abyss. Lights exploded.

Another earthquake and the room collapsed. The catwalk finally gave way. Gaster heard Dr. Betas screaming.

In the final moment as everything fell, Gaster saw Sans’s face appear in the window of the time machine, saw his fists banging against the reinforced glass.

Gaster summoned a few of his bullets and spelled out two words.

He looked down at the darkness rushing up to meet him and smiled.

**“Something needs to be done about that.”**

The world went dark and Gaster felt himself shatter.

 

***

 

Spacetime tore like fabric. Gaster spread into every corner, slipping between the tears and seams. Determination kept Gaster alive and conscious. Just long enough.

With many hands, he took hold of each scattered piece of spacetime and pulled it back. He pulled the tears closed. Held the seams together.

Spacetime could heal, but not like this. The timeline would come apart again as soon as Gaster let go.

One last thing.

He reached for something bright. Took hold of it and cracked it open. It wouldn’t be enough to Save everything. No, not everything. But that was fine. The important parts would remain.

Slowly, spacetime began to knit back together. Slowly, the timeline began to reform.

It would hold.

They would be fine without him.

Gaster let go.

 

***

 

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**Works inspired by this one:**

  * [The Scientist [Podfic]](https://archiveofourown.org/works/7992922) by [Pod Person (hissykittin12)](https://archiveofourown.org/users/hissykittin12/pseuds/Pod%20Person)




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